suppose, supposing and what if

Suppose, supposing and what if can all be used with present tenses to make suggestions about things that might happen.

• ‘I haven’t got a tablecloth.’ ‘Suppose we use a sheet.’
• ‘Let’s go to the beach.’ ‘Supposing it rains.’
• What if we invite your mother next weekend and go away the week after?

A past tense makes the suggestion sound less definite.

• ‘Daddy, can I watch TV?’ ‘Suppose you did your homework first.’
• ‘I’m going to climb up there.’ ‘No! supposing you slipped!’
• What if I came tomorrow instead of this afternoon?

In sentences about the past, past perfect tenses are used to talk about situations that did not occur.

• That was very clever, but supposing you had slipped?

note

For more about past tenses with present or future meanings, (see here).