Learn Romanian: Nu-mi pasă de reacția lor. - I don't care about their reaction.

It is a contraction of the negative word nu (not) and the dative pronoun îmi (to me). In Romanian, 'I care' is phrased as 'it matters to me', so we drop the î in îmi and attach it to nu with a hyphen to make it flow smoothly as one sound.

Why is the verb pasă instead of an 'I' form?

The verb a păsa (to care / to matter) works backwards compared to English. The subject is an implied 'it', so the verb stays in the 'it' form (pasă). The person who cares is expressed as the receiver of the action using the 'to me' pronoun (-mi). Literally, it translates to 'it does not matter to me'.

Why do we use de for 'about' here instead of despre?

The verb a păsa always pairs with the preposition de to mean 'to care about'. Even though despre often translates to 'about' in English, you cannot use it with this specific verb.

Why is it reacția instead of just reacție?

Because we are talking about 'their reaction', the noun needs the definite article ('the'). For feminine nouns ending in -ie like reacție, the article is added to the end by changing it to -ia, making it reacția.