People use language to do many different things. For example, language is used to claim something, to insult someone, to promise something to someone, to ask something, to give a command, to express surprise or to do very specific actions, like when a judge declares someone guilty or when a civil servant declares two people married. Acts that are performed linguistically are called speech acts.
LIS has developed specific grammatical constructions that are typically associated to certain speech acts: declaratives are typically used to make assertions, interrogatives are typically used to ask questions, imperatives are typically used to elicit a behaviour from the addressee and exclamatives typically convey the information that something is surprising or noteworthy. However, there is no one-to-one correspondence between sentence type and speech act, as shown below.