This numbering goes back to Greek grammarians. Here is the Τέχνη Γραμματική (Art of Grammar) ascribed to Dionysius Thrax:
πρώσοπα τρία, πρῶτον, δεύτερον, τρίτον· πρῶτον μὲν ἀφ᾽ οὗ ὁ λόγος, δεύτερον δὲ πρὸς ὃν ὁ λόγος, τρίτον δὲ περὶ οὗ ὁ λόγος.
"There are three persons ['faces'], first, second, third. The first is the one from whom the speech [proceeds]...
In addition to the other answers, on specific conditions, one can also express an indefinite person using the present subjunctive(*) of the 2nd person singular. In a manner not not unsimilar to "Generic you" in English and other languages.
According to A&G (§518) in general conditions :
The subjunctive is often used in the 2nd person singular, ...
There are a couple of related words and each has a number of possible forms:
confluere, verb, "to flow together"
confluo, "I flow together"
confluimus, "we flow together"
confluens, "flowing together" (a participle)
confluendo, "by flowing together" (a gerund)
conflux or confluens or confluvium, noun, &...
The difference is tense.
The present tense is destruitur, the perfect tense is destructa est.
Is Rome being destroyed or has it been destroyed?
Only active forms are formed from the perfect stem destrux-.
The corresponding passive forms (perfect, pluperfect and future perfect tenses) are formed with the passive perfect participle desctructus (in appropriate ...
I've read the following etymologies
-ī> Directly from the first person perfective ending *-h₂e of PIE. (from Origins of the Greek Verb by Andreas Willi, Pg 8). He states the PIE *-h₂e became -ai in Old Latin, subsequently evolving into -ī.
-isti> Probably from PIE *-s-th₂e . The /s/ aorist marker was attached to the stem, followed by Stative *-th₂e, ...
If you're using the 7th edition of Wheelock's Latin (the most recent edition), this is explained at the bottom of page 4:
Note that the stem vowel has no macron in certain forms (e.g., moneō, laudānt); learn the following rule, which will make it easier to account for macrons that seem to disappear and reappear arbitrarily:
Vowels that are normally long are ...