sentar vs sentarse
Sentarse is 'to sit down' — reflexive, which learners often drop. But sentar (without reflexive) means 'to suit' or 'to agree with' you physically.
sentarse
To sit down — always reflexive. The reflexive 'se' is not optional.
Siéntate aquí.
Sit here.
Imperative of sentarse — the reflexive is built in. 'Sienta aquí' means something else entirely.
Se sentaron en la terraza.
They sat on the terrace.
Third person plural — reflexive agrees. Never drop it.
Me senté a esperar.
I sat down to wait.
The action of sitting — then waiting. Reflexive marks it as affecting you.
sentar (non-reflexive)
To suit, to agree with physically, or to sit someone else down. Gustar-type structure for well-being.
Ese color te sienta muy bien.
That colour really suits you.
Sentar bien = to suit/become. Works like gustar — the colour is the subject. Fashion and appearance.
La comida picante no me sienta bien.
Spicy food doesn't agree with me.
Food agreeing or disagreeing with your stomach — sentar bien/mal. Very common.
Le sentó mal lo que le dijiste.
What you said didn't sit well with him.
Emotional: something sitting badly — an offence or upset. Sentar mal for things that land badly.
Tricky bit
'Siéntate' (sit down — reflexive command) vs 'sienta' which sounds similar but comes from sentar and means 'it suits.' In practice the context makes it clear, but learners saying 'sienta aquí' when they mean 'sit here' cause momentary confusion.