Learn Russian: Мой брат поздно ужинал.

This is how you form the past tense for regular verbs! You take the infinitive form ужинать (to have dinner), drop the -ть ending, and add a past tense ending. Because the subject here is брат (a masculine noun), we use the masculine ending -л. If the subject were female, it would be ужинала.

How do you pronounce поздно? Do I say the д?

No, the д is completely silent! You pronounce it as if it were written позно. Russian sometimes drops sounds in groups of consonants to make words easier to say.

Why is поздно in the middle of the sentence instead of at the end?

In Russian, adverbs like поздно (late) usually come right before the verb they describe. In English, we put late at the end of the sentence (had dinner late), but in Russian, поздно ужинал is the most natural word order.

Why is there only one word (ужинал) for had dinner?

Russian has special verbs for eating meals. Instead of saying ate dinner, you use the single verb ужинать (to have dinner). The same applies to breakfast (завтракать) and lunch (обедать).