Learn Dutch: We moeten erop letten dat we tactloos commentaar vermijden. - We must make sure that we avoid tactless comments.

The verb phrase is letten op (to make sure of / to pay attention to). Because you cannot link a preposition directly to a dat-clause in Dutch (you cannot say op dat), you must use a pronominal adverb. The op turns into erop to point forward to the subordinate clause.

The English translation says 'comments' (plural), but the Dutch sentence uses the singular commentaar. Why?

In Dutch, commentaar is generally treated as an uncountable mass noun, much like 'feedback' or 'criticism' in English. If you need to refer to countable, individual comments, you would typically use the word opmerkingen instead.

Why doesn't the adjective tactloos get an -e ending here?

Commentaar is a neuter noun (het commentaar). When a neuter noun is used indefinitely—in this case, as an uncountable noun with no article—the adjective modifying it does not get an -e ending. If you made it definite, it would become het tactloze commentaar.

Can erop be split into two words in this sentence?

Yes! While it is written as one word here, you could separate er and op if you added another word, such as an adverb. For example, you could say We moeten er goed op letten dat... (We must pay close attention that...).