Common Mistakes

• Supposing there are deer in the forest. Let's go hunting.✓
• Supposing if there are deer in the forest. Let's go hunting.✖️
• Supposing he disagrees to all our proposals, what's the second option?✓
• Supposing if he disagrees to all our proposals, what's the second option?✖️

Explanation:
We use "suppose(ing), if or what if" are used to make suggestions that something might happen, we can use them with either present or past tenses; however, a past tense is more suitable because it makes the suggestion seem less direct/definite.
Note also that we can use only one of these words at a time; we can't use them together.

• Mom, can I go swimming? Suppose you had your lunch first.
• I have decided to beat that arrogant man. No! Supposing you went to jail!

Past perfect tenses can also be used to talk about the situations that did not occur:

• That was awesome, but supposing you had injured yourself?