Acabar de, Deixar de, Voltar a

Where English uses adverbs like just, again or no longer, Portuguese often uses a verb + preposition + infinitive. Each verb takes a fixed preposition:

• acabar de = to have just done (Acabei de chegar. I've just arrived.)
• deixar de = to stop, no longer do (Deixou de fumar. He stopped smoking.)
• voltar a = to do again (Voltei a ligar. I called again.)
• começar a = to start doing
• continuar a = to keep on doing
• andar a = to have been doing lately
• ficar a = to stay doing, spend time doing

Only the first verb is conjugated; the infinitive never changes. Learn each verb together with its preposition: de goes with acabar and deixar, a with all the others. Não deixes de... means "be sure to...".