I took clarinet lessons only for 4 months and my teacher told me that I must cover and uncover the mouthpiece with the tongue between note and note. I find it difficult and I think I prefer to simply cut off the air flow and blow again, but I don't know if this is the right technique.
I am not really sure what you mean by cover and uncover. But as a clarinet player with nearly 30 years under my belt I would say there are three (four) ways of articulation.
- Legato(Slur): You keep the air flow going, just change the fingering. This is shown by the arc over the notes.
- Staccato: You stop the reed with your tongue and restart the tone by synchronized airflow and release of the reed by your tongue (the "Te-Te" tonguing). This is denoted as dots over or under the notes.
- Portato/Tenuto: The more gentle stop, done with more a De-De tonguing. Shown by dots and arcs (Portato) or small bars (Tenuto).
The answer to "Should I always.." really depends on what's in the notes. I do not have my Baermann at hand, but I will check later and update. The clarinet school of Baermann is imho the Gold Standard for this questions.
I guess your teacher was trying to get you to tongue each and every note. It's a better way than merely blowing to start each note - the result is more uniform and clearer, with a controlled start to the note, and more even playing.
Once you get into producing notes more clearly - especially their attack, you'll find that you can slur one note into another by simply changing fingers, but carry on with the blow. This makes moving from one note to the next smoother - somewhat reflecting the way we put words together in phrases and sentences.
Obviously, placing the tongue back on the reed after playing a note will stop it sounding, so again, teacher gave you more control over the end of each note too, which for a beginner makes it sound tidier.
Yes, tonguing is a necessary technique. There's hard tonguing, soft tonguing and many degrees in-between. Starting a note with minimal or no tonguing is also a valid technique, but it would be a big mistake and very restrictive to try to make it the ONLY technique you use.
Try playing this without tonguing. See what I mean?
(ENDING a note with the tongue is generally frowned upon. 'Taa...' is a pleasing sound. 'Tu...t' isn't.)
Maybe lost in translation, but you shouldn't be using your tongue to cover/uncover as if to block the air flow. It's just a light touch of the tongue against the reed to stop the reed vibration. Like when you ring a bell, then touch your finger against it to silence the ringing. It may feel unusual at first, but try lighter or stronger pressure against the reed until it feel right to you. Good luck with your music study!