Tham khảo tài liệu 'longman grammar of spoken and written english part 35', ngoại ngữ, ngữ pháp tiếng anh phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Tense 453 Tense In this section for illustrative purposes we focus on the simple or unmarked present and past tenses . those which do not have marking for aspect or voice . However the frequency data we present account for all cases of the present and past tenses including those marked for aspect and voice. Basic tense and time distinctions From a structural point of view English verbs are inflected for only two tenses present and past. However many verb phrases are not marked for tense. First tense is not marked for imperative clauses and non-finite clauses . to-clauses and ing-clauses as complement clauses see . Beyond that finite clauses can be marked for either modality or tense but not both. Thus the presence of a modal verb precludes tense marking although modal verbs sometimes express time distinctions . Verb phrases which are marked for tense but not for modality will be referred to as tensed. Present tense is unmarked morphologically except for the suffix e s on the third person singular past tense for regular verbs is marked with the suffix -ed see for an overview and for further discussion of irregular verbs . The two tenses typically refer to present and past time respectively Simple present tense referring to the present time 7 want a packet of crisps conv Some recent field experimental evidence suggests that biotic interactions also can be important to grasshoppers acad Simple past tense referring to past time Well 7 rang them up yesterday conv A few vultures looked down from the roof with shabby indifference fict We examined this question by excluding birds for 3 years from experimental plots in a semiarid grassland in Arizona acad Simple present tense referring to the present time has two major meanings to describe a state existing at the present time and to describe present habitual behavior. Simple present tense referring to a state existing at the present time 7 want a packet of crisps conv 7 think you
