The document discusses the different forms that can be used in the if-clause and main clause of a first conditional statement. The if-clause can use the present simple, present continuous, or present perfect tense, while the main clause can use will for future time, the imperative form, or modal verbs like may, can, must, or shall. These conditional statements are used to describe possible outcomes based on hypothetical present or future conditions. The same constructions are also possible using time words like "when" instead of "if".