Ne... que, Seulement, and their negatives

There are two common French equivalents for the restrictive only in English: ne... que and seulement. These two terms mean essentially the same thing, but the first is a negative adverb while the second is an adverb of quantity. Their negative forms, however, are a bit more complicated.

Ne... que is used much like other negative adverbs.*

Je n'ai qu'un livre. I only have one book.

Il ne voit que les films étrangers. He only sees foreign movies.

*Except that after other negative adverbs, the indefinite and partitive articles change to de (learn more), while with ne... que, the indefinite and partitive articles do not change: Je n'ai pas de livre vs Je n'ai qu'un livre.
  

The ne goes in its normal negative position in front of the verb. The que must be placed directly in front of the word it is qualifying. Note how the position of que can change the meaning:

Je n'ai lu que deux pages. I only read two pages.

Je n'ai lu deux pages que pour te faire plaisir. I read two pages only to please you.
The only reason I read two pages was to please you.

Il ne veut que travailler à la banque. He only wants to work at the bank (not invest there).

Il ne veut travailler qu'à la banque. He wants to work only at the bank (not at the store).
  

Seulement is used in much the same way, and is also placed directly in front of the word it qualifies.

J'ai seulement un livre. I only have one book.

Il voit seulement les films étrangers. He only sees foreign movies.

J'ai lu seulement deux pages. I only read two pages.

J'ai lu deux pages seulement pour te faire plaisir. The only reason I read two pages was to please you.
  

Negation

Ne... que has the negative version ne... pas que, while seulement has two negatives: ne... pas seulement and non seulement. These all mean not only and the first two are pretty much interchangeable and can stand alone or be followed by additional information. Non seulement, however, cannot be used in a stand-alone clause; it needs to be balanced with something like mais encore, aussi, etc., as you'll see in the following examples.

Je n'ai pas que 3 livres (j'ai 2 stylos aussi.) I don't have only 3 books (I have 2 pens too)

Je n'ai pas seulement 3 livres... I don't have only 3 books ...

J'ai non seulement 3 livres, mais aussi 2 stylos. I don't have only 3 books, I have 2 pens too.

Il n'y a pas que le travail (il faut vivre aussi). Work isn't all there is; There's more than just work.

Il n'y a pas seulement le travail.... Work isn't the only thing.

Il y a non seulement le travail ; il faut vivre aussi. Work isn't all there is; you have to live too.

Il n'était pas qu'en retard.... He wasn't just late (... there's more to it than that).

Il n'était pas seulement en retard.... He wasn't just late...

Non seulement il était en retard, mais il était encore ivre. He was not only late, but drunk (too).
Not only was he late, he was (also) drunk.