You’ve likely been constructing basic conditional sentences with the Latin word si for a while, using the present tense, indicative mood verbs. Conditionals are pretty essential and basic ways to communicate. In its essence, a conditional is a sentence with a an “if” statement (the protasis) and a “then” statement (the apodosis) that makes a statement that follows from the first clause. Note: we don’t always include the word “then” when we're writing conditionals in English but it's still implied. Here are some examples of conditionals.
If my cat walks across my laptop again, (then) I will lose my mind.
If you don't stop licking me, you'll be sorry.
We would have bought tape if we had been smart.
She’ll be stuck at home unless she gets her car fixed.
(“unless” = “if … not…” = “if she doesn’t get her car fixed”)
As you can see, in English—and in Latin—we don’t always put the “if” clause first in our sentences. In these instances, the “then” part still depends on the “if” but it’s put first in the sentence, usually to emphasize it. For example, there are probably a lot of ways that we can be smart, but in the third example, our wisdom apparently hinges on purchasing tape, so that clause is placed first. But it clearly could work in either postion.
In Latin, conditionals can use multiple moods. Most commonly, you’ll see conditionals in the indicative and the subjunctive moods. The different uses of verbal moods convey the extent to which things actually do (or will) happen if the “if” part is satisfied.
Because Latin grammar books love to categorize things and give them names, there are six basic kinds of conditionals tied to the tense and mood of the verbs of each clause.
The trick is to remember that past tense subjunctives are typically telling you that something didn’t happen (that’s where the name “contrary to fact” comes from. “If I had finished my work, I would have made cake” tells you that I was behind in my work and I did not have cake.) Whereas present tense subjunctives have a sense of future-ness to them—perhaps if something happens now, something else will happen, but that first something is still up in the air.
We all love things that fit nicely into tables, and it would perhaps be nicer if all Latin writers only ever used those six kinds of conditionals. But they don’t! Authors frequently compose sentences that mix tenses and moods. Here are some examples of mixed conditionals:
Latin authors frequently use the phrase “si quis…” or “si qui…” in conditionals. In this case, “quis” or “qu-” (in whatever case) is short for “aliquis” or “aliqu-“ = anyone, someone, or any-whatever, some-whatever (si quae puella = some girl).
si quis me ridebit, multas lacrimas fundam. If anyone laughs at me, I will pour out many tears.
si quem interfecissem, scires. If I had killed someone, you would know.
A common rhyme to help you remember this rule is “si, nisi, num, or ne? All the ali-s drop away”.