Si versus tellement

Question

Both si and tellement are used for so.  Are thee interchangeable or is there a preferred context for use?

Accepted Answer

Bonjour Kay,
There are cases where they are interchangeable but not always. Here is the rule of thumb:

si (more formal) / tellement (more informal) = so  
Il fait si / tellement froid ! = it is so cold!
C'est tellement / si bon! = it is so good!
Simon conduit si / tellement vite ! = Simon drives so fast!

tellement / tant (formal) = so much / so many
Il fait tellement / tant d'efforts = he makes so much efforts
J'ai lu ce livre tellement / tant de fois = I read this book so many times

I hope this is helpful.
Bonne journée !