"He wants to finish the book."
Translation:Han vill avsluta boken.
That's a good question - these two verbs "sluta" and "avsluta" are not synonyms, and "sluta" does not quite mean the same thing as "finish" in English. "Sluta" means to stop doing a certain task. It can be used in the imperative form to tell somebody to stop doing something immediately - "Sluta läsa! = Stop reading!". "Avsluta" is really to finish or to make complete (some work, e.g.). "Avsluta dagen = Finish the day". I hope that helps :)
A bunch of verbs (amongst them: behöver, måste etc. as well as vilja) don't have an att after them, ever. Because that's just the way it is.
Jag behöver äta - I need to eat. Jag måste gå - I must go.
You would never ever say "Jag behöver att äta" or "Jag måste att gå".
There are about twenty of these verbs and I'll try to find the list.
The rest of the verbs do, e.g.:
Jag tycker om att simma - I like to eat. Jag älskar att springa - I love to run.
Att here is the infinitive marker, which you only use with verbs when you have a "to" AS IN "to be or not to be, that is the question" (with the exception being things like vilja, behöver and måste, as mentioned above).