1. Draw a cartoon related to the sentence.

2. Draw a cartoon relating to the sentence.

3. Draw a cartoon and write down a sentence related to it below.

4. Draw a cartoon and write dwon a sentence related to it below.

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Are the sentences above grammatical?
Do we have to use 'related' or 'relating'?

They are all OK. The first and third are the best. The fourth has a spelling error (or typo).

The sentence related to the cartoon or photo or picture is called a caption. You could write this:

Draw a cartoon and a caption below it.

4. Draw a cartoon and write down a sentence relating to it below.

Thank you. #4 should be like this.

Do you mean we can use both 'related' and 'relating'?

Yes, you can. There's not much difference between them; mostly the difference is in the participial form (present or past), not in meaning.

The sentences above are not grammatically correct. The correct spellings of the words "down" and "write" are missing a letter in both sentences number 3 and 4. The corrected sentences should be:

3. Draw a cartoon and write down a sentence related to it below.
4. Draw a cartoon and write down a sentence related to it below.

As for using "related" or "relating", either option is grammatically acceptable depending on your intended meaning.

In sentence 1, using "related" suggests that the cartoon should already be portraying a subject connected to the sentence itself.

In sentence 2, using "relating" suggests that the cartoon should be created in a way that establishes a connection or relationship with the sentence.

Ultimately, the choice between "related" and "relating" depends on the specific context or intent you have in mind for the task.