Đáp án:

$1.$ Mary who lives next door is very friendly.

$→$ $who$ thế cho chủ ngữ chỉ người, làm chủ ngữ.

$2.$ My mother asked me what I was doing.

$→$ $\text{$S$ + $asked$ + $O$ + $wh-word$ + $S$ + $V$ (hạ bậc)}$

$3.$ If Phong had enough money, he could travel abroad.

$→$ $\text{$ĐKL$2: If + $S$ + $V_2$$_/$$_e$$_d$ + $O$, $S$ + $would/could/...$ + $V$ + $O$}$

$→$ Mệnh đề có "$so$" là mệnh đề chính

$4.$ If you turn off all lights, you will not pay more money.

$→$ $\text{$ĐKL$1: If + $S$ + $V_h$$_t$$_đ$ + $O$, $S$ + $will/can...$ + $V$ + $O$}$

$5.$ The man whom you met at the party last night is a famous actor.

$→$  $whom$ thế cho chủ ngữ chỉ người, làm tân ngữ.

$6.$ My classmate asked me if I had enjoyed the party the previous night.

$→$ $\text{$S$ + $asked$ + $O$ + $if/whether$ + $S$ + $V$$_q$$_k$$_h$$_t$}$

$→$ Last night →  The night before / The previous night.

$@nguyendiepphuong59391$

1. Mary, who lives next door, is very friendly.

N (người) + who + V 

2. My mother asked me what I was doing.

S + asked + O + Wh + S +V (lùi thì)

HTTD --> QKTD

3. If Phong had enough money, he could travel abroad.

If 2: If + QKĐ, S + would / could + V1 

4. If you don't turn off all the lights, you will pay more money.

If 1: If + HTĐ, S+ will/can/shall + V1 

5. The man whom you met at the party last night is a famous actor.

N (người) + whom + S + V 

6. My classmate asked me if I had enjoyed the party the night before.

S + asked + O + if/whether + S +V (lùi thì)

QKĐ --> QKHT 

last night --> the night before / the previous night 

`color{orange}{~MiaMB~}`