What if + (subject + verb)

Here you are asking a question about 'in the event of' or 'in the event that.' Usually you are looking for an answer at the time of the question that is being asked.

Here are some examples:

"What if I miss the bus?"
"What if I were late to dinner?"
"What if I called her tomorrow?"
"What if I don't understand?"
"What if someone sees me?"
"What if no one is home?"
"What if they decide to stay?"
"What if it rains while we are camping?"
"What if I do not finish on time?"
"What if we introduce ourselves first?"