F2. The Past Perfect (had left, had been etc.) (e.g. A: Was Tom there when you arrived? B: No, he had already left.)
a. The Past Simple (left, was etc.) (e.g. A: Was Tom there when you arrived? B: Yes, but he left soon afterwards.)
F3. We use 'will' with the Perfect to show that something will be complete at some time in the future. (e.g. In a few years they will have discovered a cure for the common cold.)
F4. We use other modals with perfective aspect when we are looking back from a point in time when something might have happened, should have happened or would have happened. The point of time may be in the future. (e.g. We’ll meet again next week. We might have finished the work by then.)
F5. We use 'would' with the perfect to refer to something that did not happen in the past but would have happened if the conditions had been right. (e.g. If you had asked me I would have helped you.)
Idiom 365
Here today, Gone tomorrow: It's important or present now, but nobody know in future
Example: A lot of new internet companies are here today and gone tomorrow.