Why bother, when everyone speaks English
They do, in central Lisbon and Porto, and they will switch the moment they hear you struggling. Use the Portuguese anyway. It changes the temperature of the exchange in a way that is obvious and immediate, and Portugal notices effort more than it notices accuracy.
These are the mainland versions, not the Brazilian ones, and the difference matters more than you would expect for phrases this short.
Se faz favor
Please, and effectively compulsory. Every request, however small, carries it.
It does not sound like it looks. The e in se nearly vanishes and the z in faz goes to "sh", leaving something like "s'fash fav'r". Brazilian por favor will be understood perfectly well and will also be noticed.
Ordering coffee, which is regional
You will do this more often than anything else on the list, and the word changes depending on which city you are standing in.
- Lisbon: a bica. The story goes that it stands for Beba Isto Com Açúcar, drink this with sugar, supposedly a sign in a café talking customers into the stronger roast. Probably invented after the fact, still repeated constantly.
- Porto: a cimbalino, after the La Cimbali machines.
- Anywhere: pingado for an espresso with a drop of milk, meia de leite for half and half in a bigger cup.
Obrigado, obrigada
The ending agrees with the speaker. Men say obrigado, women say obrigada, regardless of who is being thanked. Almost everyone gets this backwards at least once.
In practice the opening o disappears and you will hear "brigad'" going past you at speed.
Interrupting someone, and asking where things are
Desculpe to a stranger, desculpa to a friend. Get this one the wrong way round and you sound either rude or oddly intimate.