. Mrs .Nga has taught English for five years .

-> S + began + to + V/ Ving + O + time + ago .

=> S + has / have + Vpp + O + for + time .

2. It is very dangerous to play with matches .

-> Ving + O + tobe + adj .

=> It + tobe + adj + to - V .

3. My father used to work in this company .

-> S + don't / doesn't + V + O + any longer .

=> S + used to - V + O .

4. Mr .Smith is a good English teacher 

->S + V(s-es ) + O + adv ,

=> S + tobe + a/an + adj + N .