Both as and when can guide adverbial clauses of time, which have similar meanings and are synonyms. Have you ever used this combination? Similar to the combination of if and when, it is often used to state requirements, obligations, conditions and necessity. , which is equivalent to connecting two clauses. This is an idiom used to refer to uncertain future events, which means "when the time comes". It means:

?

At that time (used to refer to uncertain future events)

At that time (used to refer to uncertain future events)

When something happens.

When things happen,

When needed.

When needed.

At the exact time when something will happen.

When will it happen?

For example:

Most people in this city don't have cars, they just rent a house to live in? When they need it.

Most people in this city don't have a car, they just rent one when they need it.

We will let you know when your order is ready.

We will let you know when your order is ready.

Analysis: In order to make it easier to understand, this sentence can be changed to we will inform you that your order will be ready and when it will be ready.

We will update the information on our website when necessary.

When necessary, we will update the information on the website.

Whenever my boss asks, I always explain to him the progress of the project.

I always explain the progress of the project to my boss when he asks.

I never go to the cinema. I just watch movies at home when I need to.

I never go to the cinema. When we need to watch, I watch movies at home.

He always carries a pocket diary with him to write down his thoughts.

He always carries a pocket diary with him and writes down what he thinks.

We'll go there and see the results then.

We'll see the result then.

My dream is to create a virtual book, which can deliver, package and record people's learning when they need it.

My dream is to create a virtual book, which can provide, package and record personal learning when needed.

The bank stores and obtains the owner's signature when needed.

The bank will keep the owner's signature and pick it up when necessary.

Although it is difficult to explain why as and when can be combined in grammar, idioms are idioms. Anyway, just remember that as and when mean "until …".

Do you know how to use the idiom "act and be"? When needed.

You will know how to use the idiom an when you need it.