Complete the following sentence:
'If I had known you were waiting outside, I _________.

Sentence টি Perfect conditional/ 3rd conditional,
তাই clause- এ would/could/might + have + V₃ + extension হবে।
অর্থাৎ, শূন্যস্থানে would have invited you to come in হবে।

------------------------
Structure of conditional sentence:

• Zero Conditional = If + Present + Present (shows scientific and general truth)
• 1st Conditional = If + Present + Future
• 2nd Conditional = If + Past Simple + Future in Past (S + would/might/could + Base Form of the Verb)
• 3rd Conditional = If + Past Perfect (had +V3) + Perfect Modal (S + would have/could have/might have + V3)

বা clause গুলোকে উল্টে দিয়েও লিখা যায় -
• 1st Conditional = Future + If + Present.
• 2nd Conditional = Future in Past (S + would/might/could + Base Form of the Verb) + If + Past Simple. 
• 3rd Conditional = Perfect Modal (S + would have/could have/might have + V3) + If + Past Perfect (had +V3).

If মাঝে থাকলে কোন কমা বসে না, কিন্তু if প্রথমে থাকলে দুইটি clause কে আলাদা করার জন্য মাঝে কমা বসাতে হয়।