Syntactically, condition may be expressed by means of simple or complex sentences, or sentence equivalents. There are many definitions of what a conditional sentence is. Generally, from the point of view of logic, conditional sentences are formed by filling up blanks in the following universal formula:

The structure may be repeated if necessary (cf. Barth and Krabbe 1982:42–43).

Logicians and philosophers usually focus their attention on studies on the true-false value of sentences. We also owe to them the distinction between counterfactual conditionals (contrary to the fact) and indicative conditionals (no predictions about the fulfilment or non-fulfilment of the condition).

Linguists and grammarians have offered various classifications of conditionals depending on the stance they adopted. Traditionally, basing on Latin, conditional sentences are divided into three groups: real (realis), potential (potentialis) and unreal (irrealis), which may be interpreted as a distinction between always true, possibly true and never true. The most popular contemporary pedagogical and descriptive grammar books present a threefold distinction into the so-called first or open, second or hypothetical, and third or unreal conditionals. The distinction is based on verb forms and applied tenses, which means in the main clause the future tense for the first type (shall/will + infinitive), the future in the past for the second type (should/would + infinitive), and the future perfect in the past for the third type (should/would + have + past participle).