Learn Polish: Byłem wczoraj zdecydowanie silniejszy od brata. - I was definitely stronger than my brother yesterday.

In Polish, the most common way to say "than" in a comparison is using the preposition od. This preposition always requires the following noun to be in the Genitive case. Since brat (brother) is a masculine animate noun, its Genitive form is brata.

The lesson mentions using bardziej for comparisons. Why don't we say bardziej silny here?

Just like in English where we say "stronger" instead of "more strong", Polish has single-word comparatives for many common adjectives. Silniejszy is the built-in comparative form of silny (strong). You generally use bardziej (more) with longer adjectives that don't have their own special form.

How do we know who is saying this sentence if there is no word for "I"?

The verb byłem has the "I" built right into its ending, and it specifically tells us the speaker is a man. If a woman were speaking, she would say byłam. In Polish, you usually drop the pronoun ja (I) unless you want to add special emphasis.

What is the role of zdecydowanie in this sentence?

Zdecydowanie is an adverb meaning "definitely" or "decisively." Polish speakers frequently use it right before a comparative adjective to add strong emphasis, much like saying "way stronger" or "by far stronger" in English.