If you 'pretend' something is true, you know it is false, but act as if it were real. If you 'suppose' something is true, you think of it as true for the purpose of a conversation.
If you want to talk about something that is happening now, use suppose + [simple sentence in present tense]. Otherwise, use the past tense.
- Suppose we miss our flight today. How would we get home? ('miss' is in present tense)
- Suppose you owned an island. What would you do with it? ('owned' is in past tense)