Learn Portuguese (Portugal): Por isso, fiquei de comprar o livro no formato digital. - Therefore, I agreed to buy the book in digital format.

When you use the verb ficar followed by the preposition de and an infinitive verb (like comprar), it forms a special verbal phrase meaning to arrange, promise, or agree to do something. So fiquei de comprar means 'I agreed to buy' or 'I made an arrangement to buy'.

Can I leave out the word de and just say fiquei comprar?

No, the preposition de is the glue that makes this specific phrase work. Without it, fiquei just means 'I stayed' or 'I became', and you cannot attach an infinitive directly to it. You must always use ficar de + infinitive for an agreed action.

Why is fiquei in the simple past tense (Pretérito Perfeito) instead of the past continuous (Imperfeito)?

Making an agreement or deciding on an arrangement is usually a single, completed action in the past, which is why the simple past is used here. If you were talking about an arrangement you used to make repeatedly, you would use the Imperfeito and say ficava de.

Does por isso always mean 'therefore'?

Yes, por isso literally translates to 'for that', but it is the standard Portuguese connector for 'therefore', 'so', or 'that is why'. It connects this decision to buy the digital book to whatever reason was just mentioned.