Learn Polish: Wczoraj byłem w domu sam. - Yesterday I was at home alone.

The -em ending on the past tense of być (to be) tells us two things: it means 'I', and the speaker is male. If a woman is speaking, the ending changes to -am, making it byłam.

Why isn't the word ja (I) in this sentence?

In Polish, you usually drop the subject pronoun (like ja) because the verb ending already tells you who is doing the action. Since byłem specifically means 'I (male) was', adding ja isn't necessary unless you want to put special emphasis on 'I'.

Does the word sam (alone) change depending on who is speaking?

Yes! Because the speaker is male (which we know from byłem), he uses the masculine form sam. A female speaker would use the feminine form sama to match byłam. A woman's sentence would be: Wczoraj byłam w domu sama.

Why is it w domu instead of just w dom?

When you use w to mean 'in' or 'at' a location, the noun that follows has to change its ending. This is a grammar rule called the Locative case. For dom (home or house), the special location form is domu.