Learn Finnish: Me emme ymmärrä heitä. - We do not understand them.

He is the basic subject form meaning "they" (used when they are the ones doing the action). Because "them" is the target of the action here (the object of the sentence), we must use the partitive form, which is heitä.

If the sentence was positive ("We understand them"), would heitä change to a different form?

Actually, no! In Finnish, verbs related to feelings, thoughts, or ongoing mental processes (like ymmärtää, to understand) usually require their object to be in the partitive case. So, "We understand them" is Me ymmärrämme heitä. The partitive heitä is used whether this specific verb is positive or negative.

Why does the verb ymmärrä not have the -mme ending for "we"?

In a negative sentence, the personal ending moves to the negative word, making it emme (we do not). The main verb drops its personal ending entirely and just uses its negative stem. That is why we say emme ymmärrä instead of emme ymmärrämme.

The dictionary form of the verb is ymmärtää. Why is it spelled with a double "r" in ymmärrä?

This is a case of consonant gradation, a spelling change that happens inside words when you add certain endings. When finding the negative stem of this verb, the strong consonant combination rt softens to the weak combination rr.