Continual - continuous

Both adjectival forms, continual and continuous, mean without stopping or without a break. They are often used interchangeably:

• This refectory has been in continual /continuous use since the 15th Century.
• The continual / continuous croaking of the frogs prevented any sleep that night.

In certain contexts only continuous is possible because continual here would imply that breaks are possible. In these examples, there are clearly no breaks, so continuous is preferred:

• A continuous line of people stretched as far as the eye could see.
• They executed the dance in one continuous movement.
• The progress of pupils was measured through continuous assessment and not through examinations

When we want to describe things that happen repeatedly, continual is preferred:

• His continual drinking was bound to lead to liver failure one day.
• He refused to give up despite the continual warnings of his family.

continually - continuously

The adverbial forms, continually and continuously, are often interchangeable.

• She sniffed continually / continuously all the way through the film and disturbed everyone around her.

But when the meaning is clearly very often, rather than without a break, continually is preferred:

• I've got a very bad stomach upset and I'm continually running to the loo.

Here, continually is behaving as an adverb of frequency, cf. always, all the time, constantly. If we arranged such adverbs along a continuum of frequency, starting with least often and ending with most often, it would read:

• never > rarely > occasionally > sometimes > often >generally > nearly always > constantly/continually