There is present tense, which is where we all should be, they say, except that none of us have been sitting there since we got wise to roll call in the third grade. This tense describes how you are, as in ‘I am tense.’
Most of us live in the other two, either running playbacks on the past or scheduling the shit out of the future. Past tense describes how you were, as in “I was tense pretty much the whole of last year.” Future tense describes how you will be, as in ‘I will be pretty tense on Tuesday; I can just feel it.”
We also have Past Perfect or pluperfect, (I kid you not), and Future Perfect, which, sadly, does not have the equivalent fluperfect. Which is just one of the frustrating things about the rules of time.
Past Perfect describes how you had been before something else occurred to change things, as in “I had been tense all morning until my capybara, Jeremiah Johnson, was located down at the local Applebee’s.”
Future Perfect describes how you will have been unless something changes the trajectory of how you are, as in “I will have been tense now for 53 years waiting to find out if the 1969 Berkshire Alien Incident was a hoax.”
This is actually not true; I just found out about the 1969 Berkshire Alien Incident while researching this and I have been living blissfully unaware of its existence until now, (Present Perfect Continuous, if you must know).
Then we have some other magical gradients of time. We don’t need to get into all of it. Predictability can be depressing. You could study the whole lot of it if you want to become a blowhard expert in time-travel or have plans to become the clocksmith of the heart.