Latin Conditional Sentences: si, nisi and the Subjunctive

A Latin if sentence is two clauses joined by si (if) or nisi (unless). If the condition is treated as real, both verbs are ordinary indicatives. If it is imagined or untrue, both go into the subjunctive, and the tense says how far from reality you are.

The if clause is the protasis and the main clause the apodosis. You will meet the labels in grammars; you do not need them.

Plain conditions: si with the indicative

A real condition makes no claim about whether it is true. It says only that if one thing is so, another follows, and each half takes the tense its own meaning needs.

The first row is how Roman letters opened: Si vales, bene est; ego valeo, if you are well, that is good; I am well. It was so routine that writers abbreviated it to its initials.

The halves need not match in time. Si hoc dixit, errat puts the saying in the past and the being wrong in the present, exactly as English does. The proverb Si vis pacem, para bellum, if you want peace, prepare for war, is a real condition with a command.

The si clause can also come second:

• Veni, si potes. - Come, if you can.

Simple indicative conditions are core GCSE-level Latin syntax.

The future: Latin is stricter than English

English puts a future condition into the present: if you come, not if you will come. Classical Latin marks the time and normally uses the future in the si clause too.

When the si action will be over before the main one starts, Latin goes one step further and uses the future perfect: si veneris, gaudebo. The future perfect page has the forms. Either way, translate with an English present.

Nisi: unless

Nisi negates the whole condition. It means unless, or if not:

• Nisi venies, te non videbo. - Unless you come, I shall not see you.
• Nisi utile est quod facimus, stulta est gloria. - Unless what we do is useful, glory is foolish. (Phaedrus, Fables 3.17)

After a negative word, nisi means except:

• Nemo nisi rex hoc scit. - Nobody except the king knows this.
• De mortuis nil nisi bonum. - Of the dead, nothing but good. (a proverb)

Si non, sin and si quis

Si non keeps the negative on one word, usually because a contrast follows with at (at least): if not rich, he is at least happy. Sin opens a second condition against the first: if you stay, I am glad; but if you go...

After si, nisi, num and ne, the indefinite aliquis drops its ali-, which is why if anyone is si quis. Si aliquis exists, but it stresses the someone.

Conditions with the subjunctive

When the speaker treats the condition as imagined or untrue, both halves go into the subjunctive. The tense then stops meaning time in the usual way: it marks distance from reality.

One sentence, four ways:

The forms themselves are on the subjunctive page and the past subjunctives page. Here the job is to read them.

Contrary to fact: imperfect and pluperfect

These are the conditions the speaker knows are untrue. The imperfect subjunctive is about now; the pluperfect is about the past.

The facts behind them: you are not here; you were not here; you did come.

English uses a past form (were) for the present unreal, which is why learners read si adesses as past. Read the imperfect subjunctive as now, but not really.

The medieval proverb is a past unreal, aimed at someone who talked himself out of a reputation:

• Si tacuisses, philosophus mansisses. - If you had kept quiet, you would have stayed a philosopher.

One habit to recognise: verbs of being able and owing (possum, debeo, oportet) often stay indicative in the main clause, because the possibility itself was real.

• Si mansisses, te videre potui. - If you had stayed, I could have seen you.

Should and would: the present subjunctive

The present subjunctive in both halves imagines something that could still happen but is not expected. English says if you were to... or if you should..., then would. Textbooks call it the future less vivid.

• Si rex veniat, gaudeam. - If the king should come, I would be glad.
• Si quis te roget, quid respondeas? - If someone were to ask you, what would you answer?

Compare si rex veniet, gaudebo, with the future indicative: there the king is expected. The subjunctive moves the same event into suppose.

Mixed conditions

Each half takes the tense its own time needs, so the two can differ.

The first: if you had come yesterday, you would be happy now. The past did not happen, so the present is not true. The second: if he were a good man, he would not have done this. His character is a lasting fact; the deed is past.

Provided that: dum and dummodo

A proviso, provided that or as long as, uses dum, modo or dummodo with the subjunctive, and its negative is ne, not non.

Dum also means while and until, so only the sense and the subjunctive tell you it is a proviso. Modo, otherwise only, means provided that when a subjunctive follows.

How to read a condition

When you meet si, nisi or sin, do not translate from the first word. Work through it in this order:

Then choose the English: if X is, Y is; if X should, Y would; if X were, Y would; if X had, Y would have. Four English shapes cover every condition on this page.

Mistakes English speakers make

The first two are the expensive ones in a translation exam: the mood of one verb settles the meaning of the whole sentence.

What to do next

1. Learn the one-sentence, four-way table until you can change the tense on demand.
2. When you meet si, find both verbs first and name the kind before translating a word.
3. Then read on to purpose, result and cum clauses.