1. We should wash our hand before meals.

2. We should wash our hands before meals.
(Can we use 'hand' as in Sentence 1?)

3. You will do good soon.
4. You will do better soon.
(Is #3 grammatical? What about Sentence 4?)

1 answer

No, sentence 1 is unusual. Sentence 2 is correct.

Sentence 3 is incorrect. Sentence 4 is fine.
You need to use "well" instead of "good" in sentence 3 since you need the adverb form there.