Suppose & Supposing: The 'What If' Game

In most modern usage, no. They are functionally interchangeable. Some stylists find supposing to be slightly more formal or to emphasize the hypothetical nature of the scenario more strongly, but this is a very subtle distinction. You can use either one in almost any context without issue.

Yes, absolutely. You use the past perfect (had + past participle) in the suppose/supposing clause to speculate about a past that did not happen. For example: Suppose you had accepted that job in London, where would you be living now? This is the equivalent of a third conditional sentence.

Yes, when the suppose/supposing clause comes at the beginning of the sentence, it should be followed by a comma. For example: Supposing it snows, the school will close. If the order is reversed, no comma is needed: The school will close supposing it snows.

The verb to suppose on its own means 'to assume' or 'to think likely' (e.g., I suppose he's running late.). The conditional conjunction suppose introduces a 'what if' scenario to be explored. The key is whether you are stating a belief or proposing a premise for discussion.

The use of the past simple tense (e.g., Suppose you knew) to talk about an unreal present or future is a direct manifestation of the subjunctive mood. The past tense form signals hypothetical distance, not past time. Recognizing this helps explain why Suppose you will win is incorrect; the grammar requires a signal of unreality, which the past tense provides.

They are highly adaptable. Their formality is determined by the context and vocabulary around them, not by the words themselves. Suppose we grab a beer? is informal. Supposing, for the sake of argument, that the theorem is false... is highly formal. This versatility is what makes them so useful.