Dollar

[jack]

Are these correct? If not, why?
1. You will receive a ten dollar gift card.
2. You will receive a ten dollars gift card. (If this is wrong, why? Isn't 'ten' plural? Like 'You will receive a pair of gift cards." 'Pair' is plural.)

[Casiopea]

Only 1. is correct. Both 'ten' and 'dollar' function as adjectives modifying 'gift.

1. a ten dollar gift card
2. a ten dollars gift card. (not OK)

Adjective Test
Q: What kind of gift?
A: A ten dollar gift. (Adjective)

[jack]

Thanks.
Are these correct? If not, why?
1. It is five dollar cheaper. (Is 'five dollar' an adjective here? Because 'cheaper' is in here?)
2. It is five dollars cheaper.

3. It is five buck cheaper.
4. It is five bucks cheaper.

[Casiopea]

Adverb Test
Statement: It's five dollars cheaper.
Question: How much cheaper?
Answer: Five dollars.

'Five dollars' modifies the comparative adjective 'cheaper', and functions as an adverb. It is also a noun phrase in form: adjective 'Five' + noun 'dollars'. (Note that, 'dollars' requires an -s because its modifier 'Five' is plural.)