Change in states with the particle 了
The modal particle 了 has several uses. One of them is to indicate the change in the state of something or introduce a new situation.
Affirmative sentences
We use the modal particle 了 
at the end of a sentence to indicate that a situation has changed or new circumstances have appeared.
In these sentences, we usually use a predicate which is expressed by an adjective.
at the end of a sentence to indicate that a situation has changed or new circumstances have appeared.
In these sentences, we usually use a predicate which is expressed by an adjective.Examples
天冷了
It’s getting cold
tiān lěng le
突然下雨了
It suddenly started to rain
tūrán xià yǔ le
To indicate a change in the state of something, we sometimes we use the modal particle 了 with a list of verbs which don't usually combine with了 in their other meanings.
Pay attention
Here we are using 了 in the past tense or as a verb suffix.
These are the verbs:
1) verbs that express a state or do not indicate an active action. For example:
知道
在
是
在
是 Examples
我是大学生了
I am a college student now
wǒ shì dàxuéshēng le
2) verbs that express feelings and emotions. For example:
喜欢
爱
爱
Examples
我肯定你慢慢就会喜欢她了
I'm sure you'll grow to like her in time
wǒ kěndìng nǐ mànman jiù huì xǐhuan tā le
3) With modal verbs. For example:
能
会
会
Examples
我会说中文了
I can speak Chinese now
wǒ huì shuō zhōngwén le
How to use 了 in the negative sentences
To construct negative sentences, we should use the particle 不 (except with the verb 有).
We can translate these sentences as “no longer…”, “not anymore…”.
(except with the verb 有).
We can translate these sentences as “no longer…”, “not anymore…”. Pay attention
We still should use the particle 了 even when we talk about a change in the situation.
Examples
我不学法语了
I don't study French anymore
wǒ bù xué fǎyǔ le
我们没钱了
We are out of money now
wǒmen méi qián le