Can someone please help me check these for me.And help me correct them the correct way. I'LL APPRECIATED YOUR HELP :)

I have to rewrite the sentences under the grammar rule of "Punctuating Sentences" if they need no change i have to write no change.

(1) The goose bit the mailman on the leg.

My answer: No change

(2) With five gallons of gas.

My answer: tHE CAR WILL RUN WITH FIVE GALLONS OF GAS.

(3)Because the store was closed.

MY asnwer: Abigail couln't buy milk because the store was closed.

(4) For the seventh time.

My answer: Angelica please be quiet for the seventh time.

(5) The rainbow stretched across the sky form the ice cream truck to the house down the lane.

My answer:No change

(6)Between the sixth and seventh floors of the building in the stairwell over in the corner.

My answer:She was standing between the sixth and seventh floors of the building in the stairwell over in the corner.

(7) Since I started taking college classes.

My answer: My daughter misses me since I started taking college classes.

(8) She drove the car right into the garage.

My answer: She drove the car right into the garage yesterday morning.

(9) When the garage door was closed.

My answer: The day that I forgot my house is was when the gargae door was closed.

(10) Friday nights at the dinner were always busy, I prefer the activity.

My answer: On friday nights the restaurant was always busy, I prefer to stay at home for the activities.

The first 7 are good -- assuming you mean FROM (not FORM) in # 5.

Let's look at the last three --

(8) She drove the car right into the garage.

My answer: She drove the car right into the garage yesterday morning.
The original sentence is correct. You don't need to add any other words.

(9) When the garage door was closed.

My answer: The day that I forgot my house is was when the gargae door was closed.
Your answer is right, assuming you mean house KEY.

(10) Friday nights at the dinner were always busy, I prefer the activity.

My answer: On friday nights the restaurant was always busy, I prefer to stay at home for the activities.
Both the original and your sentence are run-on sentences. Also -- should "dinner" be "diner"? You could either use a semi-colon between the two clauses or you can make them two separate sentences.

What is the spelling of countries when it is plural possessive?

The spelling of "countries" when it is plural possessive is "countries'." The apostrophe is placed after the "s" to indicate possession by multiple countries. For example: "The populations of the countries' capitals vary greatly."