'Also' or 'but'

Question

I think most of the examples could be translated by keeping the negation and using 'but'. At least for me, it's clearer than losing the negation of 'ne'

For example:
Je n'ai que fait mes devoirs.  
I didn't do (anything) but my homework.

Nous n'avons regardé qu'un film.  
We didn't watch but one film.

Nous n'avons que mangé des bonbons  
We didn't (do anything) but eat sweets.

Nous n'avons mangé que des bonbons  
We didn't eat (anything) but sweets.

They're not perfect English perhaps, but with the right intonation I think they could hold in oral English.

Answer

Do not use but in a negative sense with another negative.

Incorrect: He didn't want but one good manuscript.

Correct: He wanted but one good manuscript.

Correct: He wanted only one good manuscript.

To lay emphasis and to express "only'- that person/thing/work, 'but' is used in English to the best of my knowledge. Hence the example "Je ne suis qu'allé au cinéma 
I only went to the cinema (I did nothing else)" other examples are in perfect order.