`6.` When did they (first) buy the house?

`->` How long is it + since + S + V2/ed?

`=` When + did + S (first) + V?

`7.` This is the first time I have tasted such a delicious cake in my life.

`->` S + have + never + Vpp + such + a/an + adj + Noun

`=` This is the first time + S+ have/has + V3/ed + such + a/an + adj + Noun

`8.` I have never seen such a tall man before.

This/It is the first time + S + have/has + V3/ed + such + a/an + adj + Noun 

`=` S + have/has + never + V3/ed + such + a/an + adj + Noun + before

`9.` The last time I was to Ho Chi Minh City was two years ago.

 S + have/has + not + Vpp + for/since + time

`=` The last time + S + V2/ed + was + time + ago

`10.` This is the most romantic story I have ever read.

`->` S + have + never + Vpp + such + a/an + adj + Noun

`=` This + be + superlative adj + noun + S + have + ever + Vpp

`11.` How long is it since you saw Mary?

`->` How long is it + since + S + V3/ed?

`=` When + did + S (last) + V?

`12.` How long have you worked in this factory?

`->` How long + have/has + S + V3/ed?

`=` When + did + S  + V?

`13.` We haven't decorated this room for years.

The last time we decorated this room since years.

 S + have/has + not + Vpp + for/since + time

`=` It is + time + since + S (last) + V2/ed

`=` The last time + S + V2/ed + was + time + ago

`14.` It's three months since she (last) was to the hairdresser.

 S + have/has + not + Vpp + for/since + time

`=` It is + time + since + S (last) + V2/ed

`15.` They last visited me 5 years ago.

 S + have/has + not + Vpp + for/since + time

`=`  S + last + V2/ed + time + ago