一看见这些楼房，他心里就想，自己一定要买上一套房子。
This is the sentence that I've come across. I've translated it as:
Once he sees these houses, he thinks that he can definitely buy a house himself.
But I'm always confused by 要. I seems to have several meanings... 'to want to', 'will' and I suspect, in different contexts it has some grammatical meaning.
In this context it seems to mean 'will', though I'd always learnt 要 as meaning 'want to'.
Any guidelines here?
I would translate 一定要 as "must":
Seeing those houses, he thought, "I must buy myself a house!"
要 indeed has several meanings. I'm not sure I'd call that a "problem", but it can be challenging for a learner. I think the most basic meanings are "want", "must", and occasionally "will". Just my 2 cents.
can definitely = 一定能
一定要 = must
See these two pages for a more detailed explanation:
To express "want to" one often uses the verb 想. For example I want to go to the US / 我想去美国. You can also use 要 in this sentence, but it completely changes the tone. In this case it might mean I have to the US or I really want to go the US or it can just be I'm going to the US, depending on the context. In general the tone is much stronger than for 想. For example if you want to express want to have you should use 要 and not 想.
As already mentioned 要 often indicates a firm tone. 一定 (meaning: certainly, must) makes it even stronger, so 一定要 can be translated as must.
