Explanation

Articles "A" and "AN" are used with some expressions.

For example:

Make a noise, give a warning, have a pain, have a great education etc.

Explanation

She wants to buy an apartment.

Apartment gives vowel sound.

We use an when there is vowel sound.

We use a when there is consonant sound.

A & An are articles. 

Explanation

He is a university student.

Because University gives Consonant sound not vowel. 

Explanation

The mechanic went to the mosque to make old fans.

Same purpose: no article

Different purpose: Article used

Explanation

Quarter gives consonant sound while Hour gives vowel sound.

Explanation

Here the sentence is talking about a particular birthday card.

Explanation

Articles "A" and "AN" are used with some expressions.

For example:

Make a noise, give a warning, have a pain, have a great education etc.

Explanation

The patient ate an orange ? today.

مریض نے آج ایک نارنگی کھایا۔

Explanation

Please open the window!

برائے مہربانی کھڑکی کھولیں۔

Note 1: Use "A" or "An" to talk about a person or thing unknown to your listener.

Note 2: Use "The" to talk about a person known to your listener.

Explanation

Usually Article the is used for Nationalty.

Like, The Chinese, The Japanese, The Indian, The Nepalese