Question

1. The animal eats one kilogram of plant every day.

1-1. The animal eats one kilogram of plants every day.

2. The animal eats one kind of plant every day.

3. The animal eats many kinds of plant every day.

4. The animal eats many kinds of plants every day.

5. The animal eats a liter of plant every day.

(Are they all grammatical? Do we have to use the singular form 'plant' or the plural form 'plants' in the sentences?)

Answer 1

1, 3, 5 = no

1-1, 2, 4 = yes

If I refer to one day lily, I'd be referring to one plant. But if I'm not referring to any one plant in particular, then I need to use the collective plural, plants.

http://grammar.ccc.commnet.edu/grammar/plurals.htm#collective_nouns