French language Q&A Forum

Salut! Would “trop” be accepted instead of “si” in the sentences. I used it in both the sentences in the passage but they were corrected to “si”. Is there a difference? Thank you 

I am struggling a bit with the use of these, for example "You need a new bike"  I would have thought either correct but the quiz says only besoin de is correct.  Have tried reading other threads but none the wiser.