Two questions choose the form, and neither of them is what time it is. Where are you standing in time, and how are you looking at the event from there: as one closed thing, as something still running around you, or as something already over whose result is what counts where you stand.
That is why one afternoon takes three forms. I read it puts you after the reading and treats it as a single closed event. I was reading it puts you inside the afternoon with the reading still going on. I have read it puts you here, now, holding the result. Nothing about the afternoon changed between those three sentences. Your position did, and the verb reports your position rather than the afternoon.
Eight contrasts that carry the whole grid
- I work, I am working. The simple states a habit or a fact; the continuous says it is in progress around now. English forces this choice on every present tense sentence.
- I have worked and I worked. The present perfect connects to now; the past simple closes the event off. I lost my keys says it happened; I have lost my keys says they are still missing.
- I have lived here for six years, and still do. The present perfect with for or since covers a stretch reaching the present, where most European languages use a plain present.
- I have been working since nine. The perfect continuous stresses the duration rather than the result, and it is the form most often left out by learners.
- I had already left. The past perfect exists only to put one past event before another, and it is unnecessary when the order is already clear.
- I used to smoke, and I no longer do. This is the ordinary way to state a discontinued past habit, and it has no present form.
- I am seeing him tomorrow. The present continuous is the natural way to state a fixed arrangement, and it beats will for anything already agreed.
- I will be working at nine. The future continuous says the action will already be under way at that point, which is a different claim from I will work.
Why two questions cover all of it
The verb phrase is built in a fixed nesting and each layer answers one of the two questions, which is why there is no thirteenth box waiting to be discovered. The layers cannot be reordered: it is had been reading and never have was reading, because the perfect always sits outside the progressive and the mark for past or present always rides on whichever layer ended up outermost. A modal takes that outermost slot when there is one, and that is the whole reason will and might block any tense ending from appearing.
So the twelve are a count of combinations rather than a list of twelve things to acquire. The forms you have never produced are not gaps in your knowledge. They are combinations you have never had a reason to make, and three of them you will go a decade without needing.
Where it stops: the past perfect, in the month after it is taught
Almost every learner's past perfect gets worse immediately after the lesson that introduces it. Before it, had appeared only where it was needed. After it, had arrives on every event that happened earlier than some other event, and a paragraph comes out as I had woken up, then I had eaten, then I had gone to work. Each of those is grammatical on its own and the paragraph is unreadable. It is a stall rather than an error because nothing corrects it: the meaning survives, and the reader files it under heavy writing.
What was missing from the lesson is that the past perfect repairs an order the sentence has already broken. When events arrive in the order they happened, the sequence itself carries them and nothing needs marking: I woke up, ate and went to work. You reach for had when you double back, as in the train had left when I got there, where the marking is the only thing telling the reader which event came first. In order it is noise. Out of order it is load bearing.
Reading the forms off real sentences
Tatoeba is a free sentence bank with no account and no limit, and it is the fastest way to see which forms actually arrive with which time expressions. Search for since Monday or for three years and read what comes back. It replaced the example sentences in a coursebook, which were written to demonstrate the rule and therefore cannot ever disagree with it. Sentences from a bank can, and the disagreements are the useful part.
Use it in one direction only. Take a form you are unsure of, find the time expressions that keep company with it, and learn the pair. Going the other way, hunting for a sentence that proves a rule you already believe, will always succeed and will teach you nothing.
What it looks like when it goes wrong
Two failures are recognisable at a glance. The first is a perfect with a finished time expression, as in I have seen him yesterday, which fails because yesterday closes the period and the perfect needs it open. The second is the continuous on a verb that is not an activity: I am knowing the answer, I am wanting it. Knowing and wanting are states rather than things you are in the middle of doing, so there is nothing for the progressive layer to report.
Both are choices about how the speaker is looking at the event, which is also what decides the attitude verbs standing beside the tenses, can, must, should: the verbs that add attitude rather than action, and what happens to a form when you repeat what somebody else said, which is moving what someone said back one step in time. Both sit under the rules that decide what a sentence is allowed to mean.
None of that touches the harder problem, which is that this choice has to be made in the time it takes to say the word. You can be right about every one of these given a minute, and a minute is exactly what nobody gets: getting from correct sentences to an actual conversation.