本論文探詞英語情態助動詞如must, can, may, 以及其相對之半情態助動詞be certain to, have to, be likely to以及be able to之間的語意差異。本文認為epistemic情態助動詞如may（can）及must的語意含主觀性，並以說話者為導向；而半情態助動詞be likely to及be certain to的語意含客觀性，並以言談內容為導向。與‘能力’有關的概念如can及be able to之間語意的差異在於真實性（actuality），尤其用在過去時式更明顯。多數的情態助動詞都有語用功能，這是半情態助動詞所缺乏的。就句法分布來說，情態助動詞可以與半情態助動詞同時出現，但後者仍受制於選擇限制。只要不會造成語意上的謬誤及累贅，情態助動詞即可與語意上不相對的半情態動詞共同出現。

This paper examines the semantic differences between English modal auxiliaries must, can, may, and semi-modals be certain to, have to, be likely to and be able to. It is proposed that the epistemic modals may (can) and must are subjective and speaker-oriented, while the semi-modals be likely to and be certain to are objective and discourse-oriented. Regarding the notion of 'ability, can and be able to differ mostly in actuality, especially in past tense. Most modals provide pragmatic usage, while semi-modals lack this function. In terms of syntactic distribution, semi-modals can co-occur with modals, yet they are subject to selective restriction. A modal selects a semi-modal with different modality to assure that the co-occurrence does not result in semantic anomaly or redundancy.