In English, there are three contexts for conditional sentences. The first context involves the possible outcome of an event that is likely to occur: If + Present Simple/Present Progressive + Present Simple/Present Progressive/Future Simple/Future Progressive/Imperative If + I miss the bus, + I will be late for school The second context involves the possible outcome of an event that is less likely to occur; various sentence structures are possible for this context: If + Present Perfect/Present Perfect Progressive/Preterite/Past Continuous + Full Infinitive + Conditional Present/Conditional Present Progressive If + I was + to miss the bus, + I would be late for school. In some dialects: If + Imperfect Subjunctive + Full Infinitive + Conditional Present/Conditional Present Progressive If + I were + to miss the bus, + I would be late for school. In some dialects: If + modal auxiliary "should" + Bare Infinitive + Conditional Present/Conditional Present Progressive If + I should + miss the bus, + I would be late for school. The latter two formulations, with the imperfect subjunctive or the modal construction, can be modified to use subject-verb inversion instead of the conjunction if: Were I + to miss the bus, + I would be late for school. Should + I miss the bus, + I would be late for school. The third context involves the hypothetical outcome of an event that did not occur: If + Pluperfect/Pluperfect Progressive + Conditional/Conditional Progressive/Conditional Perfect/Conditional Perfect Progressive If + I had missed the bus, + I would have been late for school. Here too the conjunction if can be replaced by subject-verb inversion: Had I missed the bus, + I would have been late for school. In each formulation it is possible to reverse the order of the clauses; however, the protasis must always follow the word "if" or exhibit subject-verb inversion: If + I miss the bus, + I will be late for school can be adjusted to I will be late for school + if + I miss the bus. Should + I miss the bus, + I will be late for school can be adjusted to I will be late for school + should + I miss the bus. Latin Conditional sentences in Latin are traditionally classified into three categories, based on grammatical structure. simple conditions (factual or logical implications) present tense [if present indicative then indicative] past tense [if perfect/imperfect indicative then indicative] future conditions "future more vivid" [if future indicative then future indicative] "future less vivid" [if present subjunctive then present subjunctive] contrafactual conditions "present contrary-to-fact" [if imperfect subjunctive then imperfect subjunctive] "past contrary-to-fact" [if pluperfect subjunctive then pluperfect subjunctive] [edit]French Si + Present de l'indicatif + Present de l'indicatif/Futur simple de l'indicatif/Present de l'imperatif Si + Imparfait de l'indicatif + Present du conditionnel Si + Plus-que-parfait de l'indicatif + Passe du conditionnel [edit]Italian Italian includes the subjunctive in the second and third formulas, and does not allow the present to mix with the future in the first formula: Se + Presente dell'indicativo + Presente dell'indicativo Se + Futuro semplice dell'indicativo + Futuro semplice dell'indicativo Se + Imperfetto del congiuntivo (subjunctive) + Presente del condizionale (or, more informal, Se + Imperfetto dell'indicativo + Imperfetto dell'indicativo) Se + Trapassato (Pluperfect) del congiuntivo + Passato del condizionale
A modal verb serves as an auxiliary to another verb, which appears in infinitive form (the bare infinitive, or the to-infinitive in the cases of ought and used as discussed above). Examples: You must escape; This may be difficult. The verb governed by the modal may be another auxiliary (necessarily one that can appear in infinitive form – this includes be and have, but not another modal, except in the non-standard cases described below under Double modals). Hence a modal may introduce a chain (technically catena) of verb forms, in which the other auxiliaries express properties such as aspect and voice, as in He must have been given a new job. Modals can appear in tag questions and other elliptical sentences without the governed verb being expressed: ...can he?; I mustn't.; Would they? Like other auxiliaries, modal verbs are negated by the addition of the word not after them. (The modification of meaning may not always correspond to simple negation, as in the case of must not.) The modal can combines with not to form the single word cannot. Most of the modals have contracted negated forms in n't which are commonly used in informal English: can't, mustn't, won't (from will), etc. Again like other auxiliaries, modal verbs undergo inversion with their subject, in forming questions and in the other cases described in the article on subject–auxiliary inversion: Could you do this?; On no account may you enter. When there is negation, the contraction with n't may undergo inversion as an auxiliary in its own right: Why can't I come in? (or: Why can I not come in?). More information on these topics can be found at English clause syntax. [edit]Past forms The preterite (past) forms given above (could, might, should and, would, corresponding to can, may, shall and will respectively) do not always simply modify the meaning of the modal to give it past time reference. The only one regularly used as an ordinary past tense is could, when referring to ability: I could swim may serve as a past form of I can swim. All the preterites are used as past equivalents for the corresponding present modals in indirect speech and similar clauses requiring the rules of sequence of tenses to be applied. For example, in 1960 it might have been said that People think that we will all be driving hovercars by the year 2000, whereas at a later date it might be reported that In 1960, people thought we would all be driving hovercars by the year 2000. This "future-in-the-past" usage of would can also occur in independent sentences: I moved to Green Gables in 1930; I would live there for the next ten years. In many cases, in order to give modals past reference, they are used together with a "perfect infinitive", namely the auxiliary have and a past participle, as in I should have asked her; You may have seen me. Sometimes these expressions are limited in meaning; for example, must have can only refer to certainty, whereas past obligation is expressed by an alternative phrase such as had to (see Replacements for defective forms below).