The sentence tells you the tense
A tense question never asks which tense sounds better. It asks which tense the rest of the sentence has already fixed. There are only three kinds of evidence, and one of them is always present:
The three sources of evidence. (1) A time marker — since 1947, over the past decade, in 1911, by 1995, to date, currently, next spring. (2) An anchor verb elsewhere in the sentence whose tense is not in question. (3) A sequence: two events in the past, one clearly earlier than the other.
Quy trình đúng là: đọc hết câu, khoanh mốc thời gian và động từ đã cho sẵn, rồi mới nhìn bốn phương án. Nhìn phương án trước là hỏng — bốn thì đều “nghe được” nếu đọc riêng chỗ trống. Đề thật gần như không bao giờ để chỗ trống mà không gài sẵn bằng chứng ở đâu đó trong câu.
Since the 1990s, radiocarbon calibration curvessubstantially, forcing archaeologists to revise several accepted chronologies.
- have been revised
- were revised
- are revised
- had been revised
Since + a starting point runs a stretch of time from that point up to now, which is the present perfect's job. (B) closes the action off in the past and clashes with since. (C) states a general habit, not a stretch. (D) needs a later past moment to look back from, and the sentence gives none. Answer (A).
Time markers that decide the answer
- in 1911, between 1910 and 1935, last century, during the war — a closed window in the past $\rightarrow$ simple past.
- since 1947, over the past decade, to date, so far, in recent years — a window that reaches the present $\rightarrow$ present perfect.
- by 1995, by the time the team arrived — a past vantage point with something already finished before it $\rightarrow$ past perfect.
- by 2031, by the time the scaffolding comes down — a future vantage point $\rightarrow$ future perfect.
- when, after, as soon as, until introducing a future clause $\rightarrow$ present, never will.
By the time the survey team reached the crater rim, the eruption nearly six hours.
- has already lasted
- had already lasted
- already lasts
- was already lasting
By the time… reached plants a vantage point in the past, and the eruption's six hours were already complete at that point. That is exactly the past perfect. (A) would measure up to now, but the vantage point is past. (D) describes an action in progress without the completed duration the sentence states. Answer (B).
Since là bẫy hai mặt. Since 1947 (mốc) đi với hiện tại hoàn thành ở mệnh đề chính; nhưng ever since Halley predicted the comet's return — mệnh đề since có động từ riêng — thì động từ trong chính mệnh đề đó phải là quá khứ đơn, vì nó chỉ một thời điểm. Một câu có thể chứa cả hai: Ever since Halley predicted the return, comets have served as tests of gravitational theory.
Present perfect versus simple past
Both describe past action. The difference is whether the sentence closes the window.
Simple past = the window is shut (In 2011 the array identified nine hundred candidates). Present perfect = the window is still open (To date the array has identified nine hundred candidates). If the sentence names a finished year, a finished period, or a dead person's lifetime, the present perfect is wrong no matter how recent the event feels.
Although the alloy had been patented in 1936, itinto commercial production until after the war.
- has not gone
- had not gone
- did not go
- does not go
The patenting is the earlier event and the sentence already marks it with a past perfect, so the second verb is the later one and belongs in the simple past. (A) opens a window to the present, but after the war closes it. (B) would push the later event behind the earlier one, which reverses the order. Answer (C).
Tiếng Việt không phân biệt “đã làm hôm qua” với “đã làm cho tới giờ” bằng hình thức động từ, nên hai thì này rất dễ lẫn. Mẹo kiểm nhanh: thêm được cụm up to now vào cuối câu mà nghĩa không đổi thì dùng hiện tại hoàn thành; thêm vào thấy vô lý thì dùng quá khứ đơn.
Past perfect: only for the earlier of two past events
The past perfect is not “more past” as a matter of politeness. It marks one past event as earlier than another past event that the sentence actually supplies. No second past event, no past perfect.
By the time the present basin took shape, the lakethree times.
- dried out
- has dried out
- had dried out
- was drying out
Two past events: the basin taking shape, and three earlier episodes of drying. The earlier one takes the past perfect. (A) leaves both events on the same plane, which the phrase by the time forbids. (D) is progressive and cannot carry the count three times. Answer (C).
Conservatorsthe panel for two years when a chance X-ray revealed a second painting beneath the surface.
- studied
- have studied
- had been studying
- were studying
The pattern had been … ing for [duration] when [past event] is fixed: an activity running for a measured stretch that is interrupted by a later past event. (D) gives the activity without the completed two years behind it; (B) measures to the present although the interruption is past. Answer (C).
Unreal past: conditionals and reported prediction
Unreal past: If the archive had digitized the letters, they would have survived. — had + past participle in the if clause, would have + past participle in the main clause. Never would have in the if clause.
Inverted form: Had the core been drilled deeper, it would have reached the ash layer. Same rule, if deleted and had moved to the front.
Future seen from the past: a past-tense reporting verb turns will into would. The prospectus predicted that the canal would link the two rivers.
The 1804 prospectus predicted that the canalthe two rivers within a decade; construction in fact took thirty years.
- will link
- would link
- would have linked
- links
Predicted is past, so the prediction's future becomes would. (A) keeps a present vantage point that the past reporting verb has already moved. (C) marks the linking as unreal, but the canal was eventually built — only the schedule was wrong. Answer (B).
Lỗi nặng nhất và cũng phổ biến nhất: viết would have ở vế if. Câu If the archive would have digitized the letters… sai hoàn toàn, dù nó rất giống cách nói. Vế if của điều kiện không thực chỉ có một dạng: had + phân từ hai.
Consistency across a long sentence
When one sentence carries several verbs, they must all sit on the same time plane unless the sentence explicitly moves the plane.
The 1975 monograph argues that the frescoes are Byzantine in origin, and its final chapterthe same claim to the mosaics of the apse.
- extended
- extends
- had extended
- will extend
The anchor verb argues is present — the standard tense for describing what a text does, whatever its date. The second verb describes the same text and must match. The date 1975 is a decoy: it dates the book, not the act of describing it. Answer (B).
Hai loại “hiện tại” hay bị nhầm. Một là hiện tại mô tả nội dung văn bản hay tác phẩm — the essay argues, the novel opens — dùng bất kể sách in năm nào. Hai là hiện tại mô tả tri thức đang còn hiệu lực — scholars still disagree. Cả hai đều bị mốc năm trong câu đánh lừa, vì mốc năm chỉ định ngày của đối tượng, không định thì của hành động mô tả.