Sentence No- 1
I must walk two miles to school every morning when I was a child.
Explanation:
The second clause "when i was a child" shows that the action happened in the past. To show the compulsion in the past, we use "Had to". Here, we need to replace "must" by "had to".
Correction:
I had to walk two miles to school every morning when I was a child.
The other way to correct this sentence is using the expression "used to". We use this expression when we want to talk about our past "habits" or "repeated actions".
Correction:
I used to walk two miles to school every morning when i was a child.
Note:
The correction using "had to" expression is more accurate, as it is used to show compulsion while the other expression "used to" does not necessarily show compulsion.
Sentence No- 2
After the storm, we had got to pick up some fallen branches.
Explanation:
The expression "have to" & "have got to" have the same meaning. Both are used in compulsive sentences. However, we do not use "have got to/ had got to" with past or future tenses. We use "Had to" whenever we want to talk about the past compulsions.
Correction:
After the storm, we had to pick up some fallen branches.
Sentence No- 3
Guitar is most popular instrument among teen age boys, but not girls.
Explanation:
This sentence has more than one mistakes. First of all, let's see its overall structure. The clause "but not girls" isn't making any sense. It means there should be a preposition to make it clear.
Popular among boys, but not among girls.
Secondly, we know that "most" is the superlative adjective which always takes article "the" before it. So, here we need to place article "the" before "most".
Thirdly, Guitar is a singular countable noun, and we need to place an appropriate article before singular countable nouns. In this sentence the appropriate article is "A". "A guitar".
Correction:
A guitar is the most popular instrument among teen age boys, but not among girls.
Sentence No- 4
You are not well and i wish you feel better soon.
Explanation:
In this sentence, there are two independent clauses and a coordinating conjunction "and" is being used to connect/join them. This is a complex sentence. Whenever we join two independent clauses, we must place "comma" before the conjunction. Here, comma is missing before "and".
Secondly, "wish" (verb) is used to express desire for something which is unlikely to happen. To express our desire for something which is likely to happen, we use "hope". So, the second mistake in this sentence is of word choice.
Correction:
You are not well , and i hope you feel better soon.
Sentence No- 5
The grass not looking green now because it is not raining since last month.
Explanation:
It seems as if the helping verb is missing in this sentence. Apparently, it should be "grass is not looking green now", but here "look" is a non-progressive verb in this sentence. Hence, we'll use "grass does not look".
Secondly, we use Since/For with Perfect/Perfect continuous Tense. So, the right choice would be Present Perfect Tense.
Correction:
The grass does not look green now because it has not rained since last month.
Sentence No- 6
The cold air made me shiver a little when I was waiting for him.
Explanation:
"When" is used when two single actions happen at the same time/one after another.
For Example:
When i get home, i take off my shoes.
For two simultaneous continuous actions, we use "while"
For Example:
While i was writing this blog, my husband was watching a movie.
In this sentence, two actions happened simultaneously at the same time. Hence, the right word choice would be "while".
Correction:
The cold air made me shiver a little while i was waiting for him.
Sentence No-7
I offered my seat to an older woman who just smile me.
Explanation:
An appropriate preposition is missing in this sentence after "smile". We may use different prepositions with "smile" depending on the nature/sense of the sentence. Here, the appropriate preposition would be "At".
Correction:
I offered my seat to an older woman who just smile at me.
Sentence No- 8
She packed her bags, carried downstairs and put the back seat of her car.
Explanation:
In this sentence, the clause "carried downstairs" seems meaningless. She carried downstairs what? So, we need to add a pronoun here which is "them" (for bags).
Likewise, the next phrase would take the same pronoun. She put "them".
Next mistake is of preposition. Here preposition is missing before seat. she put her bags in/on back seat of her car.
Correction:
She packed her bags, carried them downstairs, and put them in the back seat of her car.
Don't say: I could feel her heart to beat.
I could feel her *heart beat.* 
Note!! 
He was seen to leave the house. 
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