This paper aims to discuss English modal system and modal evolution from a perspective of language typology. This approach contains classification of English modal categories with regard to the target and the domain of modality, and description of the methods of expressing modality. Also, it includes the discussion about diachronic changes of the meaning of English modal auxiliaries, in relation to the evolution of modal categories. First, English modal categories are divided into 3 types according to their target of modality, such as participant-oriented modality, event-oriented modality, and proposition-oriented modality. They are also divided into 4 types according to their domain of modality, such as epistemic modality, deontic modality, volitive modality, and facultative modality. Second, English has the modal system in which it expresses modality through various lexical and syntactic methods. The main lexical method is to insert lexical items of modal meanings, but other lexical devices, such as modal tags and modal particles, are used sometimes. However, the most important method of expressing modality in English is to use modal verbs, like modal auxiliaries, semi-modal auxiliaries, and periphrastic constructions. The morphological method is not used in English modal system because subjunctive as a mood system is not justified. Instead, using past tense form, that is, modal past is frequently found in expressing `tentativeness` or `unreal situation`. Third, the polysemy of English modal auxiliaries is the result of their diachronic changes, which were caused by the evolution of modal categories. The evolution of modal categories is usually realized by the enlargement of their target and domain in expressing modality.