This construction: 'Subject+ne .....que' cannot be translated into negative in English.
For example: je n'aime pas que toi = I love no one but you, or I only live you.
Je ne fais rien que admirer les étoiles= I do nothing but admiring the stars= I only admire the stars.
Notice that we are not using 'pas' after 'ne' the negative.
I hope that helps.
Actually, "but" is negative, and that's my preferred way of translating "ne ... que" to retain some sense of the French. It's difficult to see how "He can only watch" could use "but", however.
I'm not a native english speaker, so why is "He can just watch." not accepted as translation?
Is this an idiom? I don't recall being taught ne que. Does it mean "only" like ne pas means "not"
it's more of a restrictive clause: which can be translated into many other forms aside from only, such as : He can do nothing but watch, I'd also argue He can just watch should work, He can only watch ... etc...