Learn English – Difference between “What is time? And “What is the time”

This is the first time I've lost my passport.
The first time I went to London, I lost my passport.

The first time is a specific time, hence the definite article. “*A first time” would imply that there are many first times.

It is also possible to say my first time, although that is mostly done when there is no verb.

This is my first time losing my passport. [Possible but not very common]

This is the first time I've been to London.
This is my first time in London.

A first time is rare. It can be used in when referring to the concept of doing something for the first time, with no specific thing in mind.

There's a first time for everything.

When referring to a specific circumstance that is encountered for the first time, you can say a first (for me).

Losing my passport was a first for me.

First-time can also function as an adjective, to mean a person who is doing something for the first time: a first-time voter (someone who is voting or will vote for the first time in his life), a first-time writer (someone who is writing his first book), …

Describe the difference between A and B. There is one difference, describe it.
Describe a difference between A and B. There may be more differences, describe only one of them.
Describe the differences between A and B. There are several differences, describe them all.
Describe differences between A and B. There are several differences, describe some of them.
Describe difference between A and B. Wrong indeed

As J.R. remarks, there is some room for interpretation in the third sentence. The writer might not expect the answerer to give all the differences, but that depends on the situation.

If I ask a general open question about a subject the student should have studied, I could ask:

What are the differences between a monarchy and a republic?

I would expect you to answer the main distinctive differences about the forms of government, and not, for instance, whether during the 3rd century ad in western South America, societies resembled a monarchy more often than a republic.

However, if I gave you a text to read and I ask you

What are the difference that the author has noticed between the red and the blue phone?

I do expect you to list all the differences that are mentioned in the text.

In general, when you use the definite article (the), you indicate that you want a description of the specific thing or things you are asking about. So not just some random difference, but the difference, the one difference that exists between A and B - or the differences, the whole set of differences, all of them.

When you use the indefinite article (a or zero, no article), you are asking to describe any difference, or differences, that apply to A and B, but not the whole set of differences that might exist.

There is a little but in this:

When someone refers to the difference between A and B, it usually does not imply there is only one difference! The difference, often with the emphasized in some way, spoken or written, means the most important difference.

Suppose I write an article about two new phones that come out. They have slightly different prices, they have a slightly different performance and the other one lasts half an hour longer on the battery.

But the difference between phone A and B is the screen: A has a 2 inch VGA-resolution screen, whereas B sports a state-of-the-art 42 inch WTFBBQXSVGA-resolution screen.