(This is a series on Phrasal Verbs. There are hundreds of phrasal verbs and many of them have no relation to the words that form them. When you are learning a new phrasal verb, I recommend learning it as a completely new piece of vocabulary, instead of trying to make sense of the two pieces of the word.)
Phrasal verbs can be really weird sometimes. There are two in English that basically mean the same thing – sit up and sit down. The English to Portuguese translation for both of these words is basically sentar-se but they cannot be used in the same context.
Sit up
Sit up is used in contrast with lie down (deitar-se) or slouch (desleixo)
You need to sip up properly to have good posture.
Hey! Sit up, it isn’t time to sleep!
Sit down
Sit down is used in contrast with standing (ficar de pé)
If you are tired, please sit down.
I don’t like to sit down on the subway, because the seats like dirty.
