[Ainka]

I use Polish (my and my students mother tongue) as I teach beginners... And I think to some point there is a need to use mother tongue to explain grammar. Of course all the instructions are in English and they have to learn them. IMHO the FCE level is agood point to use only English in class - that how I was taught. From the FCE level (B2 of Council of Europe level) I had classes with native speaker so there was no choice And at that level I was ready to be taught only in English.

[michèle 2]

I agree with susan if using L1 helps the students with learning a foreign langage then use L1, on the other hand if it hinders the students'learning, well not using L1 appears to be a better choice. Personally I use French to explain grammar points because we compare the English language to our own language and review the French grammar at the same time. For very young learners I give the instructions in French because it saves time and when my students have a good grasp of English we just use English and "ban" French!!
Personally i think that some kids can lose motivation if we use a language they don't understand a word especially when they're beginners.