In the sentence "呆在家里， 这样你就能接电话了", I can't see why "了" is used in this sentence. I study Chinese these days, and I think that I can just say "这样你就能接电话", but in this example sentence, "了" is used. I know that "了" is often used as the meaning of "completion of action" or "change of action or state". Could you help me understand the sentence correctly?
I will take advantage of this question to write a summary of the sentence-final 了 or modal 了, which occurs at the end of a sentence. [Also, the sentence-final 了 should not be confused with the perfective or aspectual 了, which occurs just after a verb and marks a bounded action.]
From Charles N. Li, Sandra A. Thompson - Mandarin Chinese: A Functional Reference Grammar, they consider the sentence-final 了 to signal a Currently Relevant State (CRS), that is, 了 marks a state of affairs with a current relevance to the particular situation. Although in the book they argue otherwise, I believe in all cases there is always an inherent change of state. We can understand:
Currently: refers to the particular situation (and not necessarily to the present or even real situation). For example:
One week ago, all the tickets were sold out ("All the tickets were sold out" is a Currently Relevant State respect to "One week ago").
If I were French, then I wouldn't be speaking such bad French ("I wouldn't be speaking such bad French" is a Currently Relevant State respect to "If I were French").
Relevance: the state of affairs given by the sentence is relevant for the speaker and the hearer in the context of the particular situation.
State: refers to the state of affairs signaled by the sentence with 了. It can be classified into the following possibilities:
("Simple") change of state: Previously the state of affair did not hold, but now currently holds (thus, it is relevant). For example:
He knows about that piece of news (he did not before).
It's already four o'clock (it was not before).
But also, the change could be about the state of knowledge of the speaker or the hearer. For example:
It's raining (either it was not raining before or the speaker has just realized it).
Q: 她怎么样？A: 她做教授了。
Q: How is she? A: She has become a professor (the first person did not knew about this state).
Corrects a wrong assumption: The current situation contradicts an assumption. For example:
[A kid who does not want to eat his/her food, but the same food as the adults. Her mother says:]
It's the same！(You are wrong in thinking you are eating something different).
Reports current progress: the speaker uses 了 to show concern for the progress of an state. For example:
[School official registering children:]
How old are you?
How old are you?
What happens next: the current state of affairs is relevant for what can happen on the following.
[Someone is arriving:]
He is coming in.
Announces the finality of a statement: Introduces the statement into the current situation. Otherwise the sentence might sound incomplete, requiring some other justification to make it relevant. For example:
[As an answer to why the speaker did not study in a university:]
Because the tuition fees are too expensive there (the situation was already introduced).
[One student complaints to another:]
The tuition fees are really too expensive!
Judging from this webpage, it's a translation from the English:
Stay home so you can answer the phone.
In this context, they've added the change of state 了 to refer to a future change of state: it's being said to someone who will change from
- State 1: not being at home and therefore not being able to answer the phone, to
- State 2: being at home and therefore being able to answer the phone.
Dropping the 了 would be appropriate if there were no change of state involved (e.g., the person is already at home).
This dictionary definition addresses the use of 了:
Basically, it says: used in the middle or end of the sentence, denoting new circumstances or changes take place(the new circumstance could be something already happened, will happen, or just a hypothesis)
Your example is a hypothesis. If you stay home, you would be able to pick up the phone call.
Other examples for the usage: 天要下雨我就不出门了。要是不走我就见到他了。
The final particle 了 has been discussed in one of my previous post. Let's recap as follows:
The final particle 了 is the trace of an operation whereby the time frame T associated with the speaker/co-speaker (i.e. the hic et nunc) is susceptible of locating a process P, and that T indeed locates/will locate P. In practical terms, the process P is "expected" to happen ("expected" is a bit too strong, but I can't find a better term to make it easy to understand). For example:
1- 她昨天看到姚明。- She happened to see Yao Ming yesterday.
1a - 她昨天看到姚明了。- She finally got to see Yao Ming yesterday.
2 - 我要回家。- I want/need to go home.
2a - 我要回家了。- It is time for me to go home.
In (1a) and (2a), the process is/was expected in one way or another, which is not the case in (1) and (2).
In "呆在家里，这样你就能接电话了", the process "你接电话" is expected. You are waiting for a phone call, and you are not sure how "you receive the phone call" can happen. I tell you what: you stay at home, then it will happen.