"But When" in Arabic

[Pacifica]

I translated "I saw a light in the house, but when I looked (in) I did not see anyone" as:

رَأيْتُ نورا في البيتِ ولٰكنّ إذ نَظَرتُ ما رَأيْتُ أحَدا

The answer key has the same but with وإذا instead of ولٰكنّ إذ.

What I'd like to know is if the combination I used is wrong or if both are valid alternatives.

I'm not sure whom else I could bother than @Clemens ...

 

[Pacifica]

I don't deserve a like for this...

 

[Clemens]

لٰكنّ triggers syntactic changes like إنّ; the subject must come right after it and be in the accusative, or be a pronoun clitic on lakinna: لكني.

 

[Pacifica]

I see. شكرا

 

[Clemens]

These things are tricky even for native speakers, by the way. It's a little bit like asking an Italian to explain the difference between ut, ne, and ut non.