The sentences with a verbal predicate we have encountered before usually state facts without emphasizing any change of state or completion of the action. To indicate a change in state (as learnt in Unit 9) or to mean that the action (has) happened, the particle "了" is often used. As a particle, "了" itself has no fixed meaning. Depending on the context, "了" can indicate different meanings. The comparison of "了" indicating a change of the status and "了" indicating that the action (has) happened is shown below:
In A, the "了" means that the person in the conversation could not go to class before, but today the situation has changed and he can go to class, while in B, the "了" means that the speaker has already gone to school for lessons.
To further demonstrate the use of the particle "了" at the end of a sentence expressing something (has) happened, here is another comparison of questions without "了" (in C) and with "了" (in D):
In C, the questions are asking for situations that have not yet happened, while in D, the questions are about events in the past or events that (have) happened. To answer the questions with "了" at the end of the sentences in D, you can simply replace the question words with your answers. To form the negative form of a sentence with the particle "了", you need to add the adverb "méi (yǒu) 沒（有）" before the verb, and simultaneously remove "了".
Note that "沒（有）" could not be replaced by "bù 不" here. That is, you cannot use "不" before that verb to answer the questions with "了" in this case.
To ask a question, put "ma 嗎" or "沒有" at the end of a sentence：
Note:
The particle "了" can be used to indicate that an action or event has occurred in the past. However, it is not always necessary to use "了" for past events. If the statement is a general description or part of a series of events, and does not require emphasis on the confirmation of the occurrence, then "了" can be omitted. It is also unnecessary in descriptions of events that occurred at a specific time. For example,
A pivotal sentence is a sentence with a verbal predicate composed of two verbal phrases, where the object of the first verb is also the subject of the second verb. The first verb in a pivotal sentence is often a verb with the meaning of asking or ordering someone to do something, such as "请" or "让".
Both "请" and "让" can express the meaning of “to ask someone to do something”, but "请" is considered more polite.
When asking someone a question using "……，shì bu shì 是不是？" or "……，shì ma 是吗？" (..., don't you?), it often expresses an assumption. This is similar to the structure "……，duì ma 对吗?" (…, right?) learnt before. In response, if you agree, say "shì a 是啊" (Yes); if you think that’s not true, you can use "bù 不" or "bú shì 不是" (No). For example:
"是不是" may also be placed before a verb or verb phrase. For example:
