O amor é cego (eng. Love is blind, liet. Meilė akla).
This expression says, that when person falls in love with another person, he doesn’t notice things, that may annoy him. It means that you are attracted to a person and some of his manners or habits, which shade others. Also this means, that you do everything to know this person, don’t take attention to things going around you, even some stupid things.
O fruto proibido é o melhor (eng. Forbidden fruit is the sweetest, liet. Uždraustas vaisius yra skaniausias).
It means that if you forbid someone to do something, that person will want to do it even more. It is just like with children, then you forbid them to eat sweets, they want them even more. This expression is come from the Bible, then Adam and Eve were banished from heaven, for eating the forbidden fruit.
Na hora da necessidade é que se conhece o amigo (eng. A friend in need is a friend indeed, liet. Tikrą draugą bėdoje pažinsi).
This expression is usually used, then you want to separate real friend from friend, who is just using you, or who just pretend being your friend. It means, that real friend will always help you, when the help is needed, he will never turn a back on you. Also you can count on him.
Portuguese have lots of different expressions and I don’t think, that Lithuania have that much. It is good, then you can enrich your speech with proverbs, because the speech then is more live and involving. In Portugal there are a lot of proverbs, that is not used in Lithuania and this is more interesting to learn about other culture.