1. I read the
caption when i watch TV. :(
==> Each line of dialogue has the same caption (the same sentence).
2. I read the
captions when I watch TV. :D
==> Each line of dialogue has its own caption (its own sentence)
3. I read the
caption a lot when I watch TV. :(
==> I read the same sentence over and over again.
4. I read the
captions a lot when I watch TV. :D
==> I read different sentences.
5. What do these mean? :D
6. What do these means? :(
==> 'mean' is the verb and 'these' is the subject. Both are plural e.g.,
Plural:These mean.... (OK)
Singular: This means (OK)
Note that, in the singular, and with question formation, the -s is moved from the verb to the auxiliary verb 'do':
Question, Singular: What doe
s this mean?
Statement, Singular: This mean
s
7. These means :( Error: Plural subject + Singular verb
8. These mean :D Plural subject + Plural verb

9. This means :D Singular subject + Singular verb

10. This mean :( Singular subject + Plural verb
All the best, :D