the main clause -should/would + the infinitive the subordinate clause - The Past Simple Tense In your sentence we must use The Past Participle only because the subordinate clause is used in the Passive Voice.

1st one

The first one If you were bitten

Ramin, Susanne is right. The first sentence is correct. What would you do if you were bitten by a snake ? "What would you do" is the main clause. We use would/should and the infinitive. " if you were bitten by a snake" is a subordinate clause. We use The Past Simple Tense. In our example it is The Past Simple Passive consisting of were + Past Participle bitten. If we invited Ben (to the party), we'd have to invite his friends too. In this sentence the subordinate clause is "If we invited Ben (to the party)" The predicate is used in The Past Simple Tense "invited" The predicate in the main clause is "would have" - would + the infinitive "have".

"If we invited Ben (to the party), we'd have to invite his friends too." --This sentence is wrong. It changes tense after the comma. Corrected versions: If we invite Ben, we have to invite his friends too. If we invited Ben, we'd have had to invite his friends too. 'would have' is a modal auxiliary. The main verb is 'invite' in this clause. 'Invite' needs to match tense in both halves of the sentence. This is my understanding. I hope it is helpful.