1. A beaver tail is a big,flat bread.

2. A beaver tail is big, flat bread.
3. Beaver tails are big, flat bread.
4. Beaver tails are a big, flat bread.
5. Beaver tails are big, flat breads.
6. Beaver tails are big and flat.
7. beaver tails are big, flat.

(Which expressions are grammatical?)

1. A beaver tail is a big,flat bread. no

2. A beaver tail is big, flat bread. no
3. Beaver tails are big, flat bread. no
4. Beaver tails are a big, flat bread. no
5. Beaver tails are big, flat breads. no
6. Beaver tails are big and flat. yes
7. beaver tails are big, flat. no

You could make 1-5 correct by writing the sentence like this:

Beaver tails are like big, flat pieces of bread.

Based on standard English grammar rules, the following expressions are grammatically correct:

2. A beaver tail is big, flat bread.
3. Beaver tails are big, flat bread.
5. Beaver tails are big, flat breads.
6. Beaver tails are big and flat.
7. Beaver tails are big, flat.

Explanation:
1. The phrase "a big, flat bread" implies that there is only one kind of bread known as a "beaver tail," which does not match the context. It should be "beaver tail" (singular) or "beaver tails" (plural).
2. This expression correctly states that there is a singular beaver tail, which is a big and flat bread.
3. This expression correctly states that there are multiple beaver tails, all of which are big and flat bread.
4. This expression is grammatically incorrect because it combines the singular "beaver tails" with the singular article "a."
5. This expression correctly states that there are multiple beaver tails, all of which are big and flat breads (plural).
6. This expression corrects the article usage and states that there are multiple beaver tails, all of which are both big and flat. The conjunction "and" connects the two adjectives.
7. This expression corrects the capitalization of "beaver tails" but is otherwise the same as expression 3, making it grammatically correct.