I.

1. have gone ( since yesterday-thì QK nên go --> have gone )

2. would visit ( following summer ~ the next summer thì tlai nên will --> would)

3. to volunteer (decide + to V nguyên thể: quyết định làm gì đó) 

4. helping (intertested in + V-ing: thích làm gì đó) 

5. to explain/ explain (help có thể đi với to V hoặc V nguyên thể)

II. 

1. He has studied Spanish for six years. (qkđ -> htht)

2. Tom enjoys playing football every afternoon. ( enjoy + V-ing: thích làm gì)

3. My teacher advised us to buy that book. ( advise sb + to V = advise + V-ing)

4. The last exercise is not as difficult as this one. ( so sánh không ngang bằng)

5. He wanted to know if I wanted to go home early then. ( câu gián tiếp now -> then, Do-> if, you-> I) 

I)

1. have gone

DHNB: since yesterday (HTHT)

S+ has/have+ V3/ed 

2. would visit

S + said + that+ S+  V(lùi thì)

will --> would 

next summer --> the next summer / the following summer

3. to volunteer

decide + to V: quyết định làm gì 

4. helping

be interested in + Ving: thích

5. to explain

help + O + to V: giúp ai làm gì

II)

1. He has studied Spanish for six years.

S + has/have + V3/ed + for/since + time 

--> S + started / began + to V / Ving 

--> It is / has been + time + since + S+ V2/ed 

2. Tom enjoys playing football every afternoon.

enjoy+ Ving: thích 

3. My teacher advised us to buy that book.

S + advised + O + to V: khuyên 

4. The last exercise isn't as difficult as this one.

S+ be (not) + as + ADJ + as+  O : ss bằng

5. He wanted to know if I wanted to go home early then.

S + asked / wanted to know / wondered + (O) + if/whether + S + V (lùi thì)

HTĐ --> QKĐ
now --> then 

`color{orange}{~LeAiFuRuiHuiBao~}`