Does the target sense that it has been marked when it is casted or no?  Looking for your opinions please!

Think that one is up to the DM. But I would say personally that it depends on what was marked and how skillfully it was done. But nothing on the flavor text or otherwise indicates that they would. But on the off-hand the same could be said on the other way around:p

Is there a certain level your ranger needs to be before you can add this because it wont let me add any spells to my ranger.

Level 2. If you're editing your character, in the class section, right under the name of the class should be a menu you can open for spells if you're already at a level where you can cast spells

Does it work for ranged weapons or only melee?

It says weapon attack, that would mean ranged or melee.  

Is there any reason that this shouldn’t count for melee attacks? I’m debating making a Vengeance Paladin with the new unarmed fighting style.

Nope. It works with any weapon attack (which includes unarmed attacks). Have fun!

Under new variant rules for ranger, can one cast another concentration spell while hunters mark is at play? 

This is one of the best ranger spells in the core book.

This spell doesn't make sense to me

"You choose a creature you can see within range and mystically mark it as your quarry" ATTACK/SAVE None Does this mean you can cast this undetected? If not why does it last 1 - 24 hours with a range/area of 90 ft. Doesn't the spell dissipate after the target leaves 90 ft? Why would you have advantage on any Wisdom (Perception) or Wisdom (Survival) check you make to find it it it's only 90 ft area.

The range is when you first cast the spell, so the enemy has to be within 90 feet when casting the spell in order to affect them. There's nothing that says that the spell stops working if they get more than 90 feet away.

Creatures can use the hide action to try and hide. For instance goblins could try to hide as a bonus action every turn and then use attacks from unseen enemy to wear down the adventurers.

I get that the spell requires concentration so that you don't crazy stack bonuses.  But for the life of me, I can't figure out why on earth you would cast this as a higher level spell to increase the duration.  I mean if you use a 3rd level spell slot to extend it's duration to 8 hours (or God forbid a 5TH level spell slot to extend it to 24hours), all that needs to happen is a lucky kobold arrow and a bad concentration save and it's all over.  I mean do you expect not to get hit in the next 8 (or 24) hours?  There are MUCH better 3rd, 4th and 5th level spells to use over Hunter's Mark to get an extra 1d6 until you fail a concentration save.  The extended duration element (for a spell that can be broken by a failed concentration save) by using a higher level spell slot is, in my opinion, stupid.  Like the only way this would be useful is if you don't plan to get into battle during this period and you just want to get Advantage when tracking... yeah, could be useful, but for a 3rd, 4th or 5th level spell slot?  Seems pretty expensive just for Advantage on a tracking check.  Or am I missing something?

I mean I get the whole concentration check thing if you are a wizard, behind the front lines being all "avoiding getting hit" and stuff like they are apt to do.  But if you are a melee ranger, digging in on the front lines hitting and getting hit, you are GOING to get hit and eventually lose concentration within 2-3 hits.  If you are going 8-24 hours without getting hit 2-3 times, what the hell are you doing?  This spell is not going to last more than 1, maybe 2 encounters (if you are really lucky).

Sure, it's a first level spell and at 11th level I have 4 spell slots for these per day, so if you get debuffed halfway into an encounter, no big deal, cast it again.  But please don't insult my intelligence as tell me that you are going to be able to maintain concentration for a spell that gives you combat/damage related buffs for 8-24 hours!  So lame… again, in MY opinion.  I could be wrong.

Quote from JNevins234 >>

I get that the spell requires concentration so that you don't crazy stack bonuses.  But for the life of me, I can't figure out why on earth you would cast this as a higher level spell to increase the duration.  I mean if you use a 3rd level spell slot to extend it's duration to 8 hours (or God forbid a 5TH level spell slot to extend it to 24hours), all that needs to happen is a lucky kobold arrow and a bad concentration save and it's all over.  I mean do you expect not to get hit in the next 8 (or 24) hours?  There are MUCH better 3rd, 4th and 5th level spells to use over Hunter's Mark to get an extra 1d6 until you fail a concentration save.  The extended duration element (for a spell that can be broken by a failed concentration save) by using a higher level spell slot is, in my opinion, stupid.  Like the only way this would be useful is if you don't plan to get into battle during this period and you just want to get Advantage when tracking... yeah, could be useful, but for a 3rd, 4th or 5th level spell slot?  Seems pretty expensive just for Advantage on a tracking check.  Or am I missing something?

I mean I get the whole concentration check thing if you are a wizard, behind the front lines being all "avoiding getting hit" and stuff like they are apt to do.  But if you are a melee ranger, digging in on the front lines hitting and getting hit, you are GOING to get hit and eventually lose concentration within 2-3 hits.  If you are going 8-24 hours without getting hit 2-3 times, what the hell are you doing?  This spell is not going to last more than 1, maybe 2 encounters (if you are really lucky).

Sure, it's a first level spell and at 11th level I have 4 spell slots for these per day, so if you get debuffed halfway into an encounter, no big deal, cast it again.  But please don't insult my intelligence as tell me that you are going to be able to maintain concentration for a spell that gives you combat/damage related buffs for 8-24 hours!  So lame… again, in MY opinion.  I could be wrong.

The Class Variant Options in last year's Unearthed Arcana fixes this, Rangers get it for free, no spell slot, doesn't count against your prepared spells, and best of all, doesn't use concentration.

Quote from JNevins234 >>

I get that the spell requires concentration so that you don't crazy stack bonuses.  But for the life of me, I can't figure out why on earth you would cast this as a higher level spell to increase the duration.  I mean if you use a 3rd level spell slot to extend it's duration to 8 hours (or God forbid a 5TH level spell slot to extend it to 24hours), all that needs to happen is a lucky kobold arrow and a bad concentration save and it's all over.  I mean do you expect not to get hit in the next 8 (or 24) hours?  There are MUCH better 3rd, 4th and 5th level spells to use over Hunter's Mark to get an extra 1d6 until you fail a concentration save.  The extended duration element (for a spell that can be broken by a failed concentration save) by using a higher level spell slot is, in my opinion, stupid.  Like the only way this would be useful is if you don't plan to get into battle during this period and you just want to get Advantage when tracking... yeah, could be useful, but for a 3rd, 4th or 5th level spell slot?  Seems pretty expensive just for Advantage on a tracking check.  Or am I missing something?

I mean I get the whole concentration check thing if you are a wizard, behind the front lines being all "avoiding getting hit" and stuff like they are apt to do.  But if you are a melee ranger, digging in on the front lines hitting and getting hit, you are GOING to get hit and eventually lose concentration within 2-3 hits.  If you are going 8-24 hours without getting hit 2-3 times, what the hell are you doing?  This spell is not going to last more than 1, maybe 2 encounters (if you are really lucky).

Sure, it's a first level spell and at 11th level I have 4 spell slots for these per day, so if you get debuffed halfway into an encounter, no big deal, cast it again.  But please don't insult my intelligence as tell me that you are going to be able to maintain concentration for a spell that gives you combat/damage related buffs for 8-24 hours!  So lame… again, in MY opinion.  I could be wrong.

 You don't even have to avoid getting hit, you just have to make the concentration check. For a spell that allows you to choose new targets every round, 8-24 hours duration makes it a lot more potent. Considering that a Ranger doesn't even get 5th level slots until tier 4, there's a pretty good chance of getting to use it to its full extent with a character that powerful.

I feel like 90 feet is a really short range for this specific spell?

the hunter in our group uses a heavy crossbow and he has sharpshooter, which means no disadvantage up to 400 feet. Now sure, that is a few different combos at play, but even so, I feel like "within sight" for the way this spell works would be more overall useful.

It seems like it is not just for the extra damage, but also for tracking a target, and getting within 90 feet usually means you are risking being seen as you get closer.

Is that a "balance" issue? Because I dunno, I just don't think it would be unbalanced if it had a longer range.

Does 'subsequent' mean next turn or any turn after so long as the concentration is still in effect?

Quote from CaelVtroz >>

Does 'subsequent' mean next turn or any turn after so long as the concentration is still in effect?

 I would like to know this as well...