Explanation

We had met before we began working together.

The sentence is in the past perfect tense ("had met")

This indicates that the action of the meeting happened before another past action. 

Explanation

The complete sentence is:
"The teacher continued me on my spelling."

The preposition "on" is used to indicate a subject or focus,

Explanation

He was born in December 12, 1990.

وہ 12 دسمبر 1990 کو پیدا ہوئے۔

For month, year, and season, we use in.

For month, year, and season, we use in.

For example in August, in 1992, in the '90s, in winter.

For date and day, we use on.

For example 5 of August, on the fifth, on Monday.

Explanation

I enjoy playing tennis.

میں ٹینس کھیل کر لطف اندوز ہوتا ہوں۔

The verb "enjoy" is followed by a gerund (verb + ing).

So "playing" is correct.

Explanation

"We use 'will purchase' to express your intent to buy something in the future if you have the money."

 The modal verb 'will' is used to express future tense,

while 'purchase' is the base form of the verb used to describe the action of buying something.