Answer:

1. Are they preparing for Teachers' Day ?

Tobe + S + V-ing + for + N + O ?

2. What are they going to do on Sports Day ?

What + tobe + S + V-ing + N + O ?

3. Is your brother going to sing a big song on Children's Day ?

Tobe + N + V-ing + O ?

4. He is practising playing guitar in his room.

S + tobe + practising + V-ing + N + O 

5. Vovinam is a type of martial art in VietNam

N + tobe + O 

6. To do all sports, you have to well at practice a lot

S + V + PRACTICE + O 

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`1`. Are they preparing for Teacher's Day?

`-` Tobe + S + V-ing + o?

`2`. What are they going to do on Sports Day?

`-` Wh(question) + tobe + S + V-ing + O?

`3`. Is your brother going to sing a big song on Chuldren's Day?

`-` Tobe + S + V-ing + O?

`4`. He is practising playing guitar in his room

`-` S + tobe + V-ing + O

`5`. Vovinam is a type of martial art in Vietnam

`-` Tobe : N(số ít) + is

`6`. To do all sports, you have to well at practice a lot

`-` S + V(s/es)