Learn Polish: Niestety nie mógł przyjść, ale miał ważny powód. - Unfortunately he couldn't come, but he had an important reason.

Yes! The verb móc (to be able to) has a vowel change in the past tense. For a masculine subject like "he", it uses ó, becoming mógł. But for feminine and neuter subjects, as well as plurals, the stem uses a regular o. So if "she" couldn't come, it would be nie mogła przyjść.

The verb miał means "he had", coming from the dictionary form mieć. Why did the e change to an a?

Just like móc changes its vowel in the past tense, mieć also has a stem alternation. Before the masculine -ł ending, the e changes to an a, making it miał. If we were talking about a group of men, the vowel would change back to an e, becoming mieli.

How do we know the sentence means "he" couldn't come if the word on (he) is missing?

In Polish, the subject pronoun is usually dropped because the verb gives it away. The past tense ending -ł in both mógł and miał tells us that a single male is the subject. If you added on, it would sound unnaturally emphatic.

Why use przyjść (to come) instead of a basic walking verb like iść?