Learn Polish: Marek powiedział, że miał nam to wytłumaczyć, ale nie miał czasu. - Marek said that he was supposed to explain it to us, but he didn't have time.

In the first part, miał is followed by the infinitive wytłumaczyć. When mieć in the past tense is paired with an infinitive, it expresses an obligation or unfulfilled expectation, translating to 'was supposed to' or 'was meant to'. In the second part, nie miał czasu, it stands alone without another verb and simply means 'did not have'.

Why do we use the order nam to?

nam is the dative case of my (we), meaning 'to us'. When a sentence has two short pronouns like this, the indirect object (the person receiving the action, nam) typically comes right before the direct object (the thing being acted upon, to).

Why is the word for time czasu instead of czas at the end of the sentence?

This is a good reminder of the genitive of negation. Normally, having something takes the accusative case (which would just be czas). However, because the verb is negated here (nie miał), the object must switch to the genitive case, becoming czasu.

Could I use the imperfective verb tłumaczyć instead of wytłumaczyć here?