as or while

We can use as or while to talk about two longer actions that are in progress at the same time:

• There was a lot to do. While I cleaned the car, my wife was preparing lunch.
• She then did the ironing after lunch as I cleared away the dishes.

As a general rule, we tend to use while here rather than as because as has many different meanings and uses. It could be confusing if as meaning while could be mistaken for as meaning because:

• As I was doing my homework, my mum prepared my supper. (As = because)
• As I was doing my homework, my mum prepared my supper. (As = while)

as or when

We use as or when to talk about two short events that happen at the same moment. As and when are often used with just in this context. We cannot use while here:

• The telephone rang just when / just as I was about to leave. I decided not to answer it.

However, if we want to say that when one thing changes another changes at the same time, when one is the consequence of the other, we tend to use as:

• As the day wore on, it became hotter and hotter.
• As you get older, it becomes more and more difficult to make friends.

while or when

In more formal speech and writing, it is possible to leave out subject + be with when and while when main and subordinate clauses refer to the same subject. We cannot use as in this way:

• When making cranberry jam, remember to use as much sugar as fruit.
• When you are making cranberry jam, be sure to use as much sugar as fruit.
• While in France, he grew particularly fond of all varieties of cheese.
• While he was in France, he grew particularly fond of all types of cheese.

as long as: expressing time

The as ... as construction is used when we are making comparisons and comparing ideas of similar magnitude or duration

• There was extra time, so the football match lasted as long as the concert.
• He worked for as long as he wanted to on the project.
  "Take as long as you like," they said. "There's no hurry!"
  
• As long as I live, I shall smoke no more cigarettes.

as long as: expressing condition

Note that as long as is also used in conditional sentences as an alternative to provided, meaning if and only if. So long as is also possible in this context:

• I don't mind. You can leave early, as long as you finish the work.
• I don't mind. You can go home early, so long as you finish the work.
  
• I don't mind. You can leave after lunch, provided you finish all the work.

on a ... basis

The noun basis suggests a particular method or system for organising or doing something. We have the expressions on a/an hourly/daily/monthly/annual/temporary/permanent basis:

• These toilets are checked for cleanliness on an hourly basis
• She thought she would have the job on a permanent basis, but it turned out to be temporary.
• This place is known as 'the windy city' and typhoons are expected on a regular basis.

on the basis of / as a basis for

Here we have two further expressions with basis with a slightly different meaning. Used with the preposition on, method or system is suggested. Used with the preposition as, ideas, facts or actions from which something can develop is suggested:

• The contract was awarded on the basis of cost more than anything else.
• These preliminary talks will be very useful as a basis for further negotiations.