"Darling, I want you to be happy."
Translation:Kulta, haluan, että olet onnellinen.
The Finnish sentence word by word: "Darling, I want that you are happy."
Että connects two sentences or makes it possible to use a full sentence as the grammatical object of the sentence. If the object Y in "I want Y" is a sentence, it is put in its place by using "että". Furthermore, we don't omit "että" unlike English "that". For example, "I know that she is pretty" is always "Tiedän, että hän on nätti" even though you can shorten it to "I know she is pretty" in English.
"Olet" is "you are".
"Olla" is the basic form of the the verb "to be". It then changes e.g. depending on the person. With 1st and 2nd person pronouns you can leave the pronoun out.
(Minä) olen - I am
(Sinä) olet - you are
Hän on - s/he is
Se on - it is (also people in spoken Finnish)
(Me) olemme - we are
(Te) olette - you are
He ovat - they are (people)
Ne ovat - they are (animals, things, also people in spoken Finnish)