就 is used in both contemplated and completed actions, expressing a plan, request, command, statement or condition.

a. Indicates the immediacy of the next action.

    S1  V1 了 O1， (S2)  就 V2 O2   (了/吧)

    我  下  了  课，          就 回  家    了。

    As soon as I got out of class, I went home.

    Wǒ xià le kè, jiù huí jiā le.

b. Indicates an action takes or took place sooner or earlier than expected.

    S  Time-expression    就   VO ( 了)

    我    今天五点钟         就   起来了。

    Today I got up at five o’clock. 

    Wǒ jīntiān wǔ diǎn zhōng jiù qǐláile.

c. Means ” (if) …… then”

   (要是) S1 V1 O1  ， (S2)  就 V2O2

    要是  我  有 时间，  我    就  参加。

    (If) I have time，then I will attend.

    Yàoshi wǒ yǒu shíjiān, wǒ jiù cānjiā.

d. Means “… will …    =>    … 就(会, 要) + verb …

    rúguǒ míngtiān xiàyǔ, yīnyuèhuì jiù huì qǔxiāo.

    如果明天下雨, 音乐会就会取消.

    If it rains tomorrow, the concert will be cancelled.

e.The structure “就+verb”indicates a conclusion or a resolution made on the basis of what’s been  mentioned previously. For example:

If you don’t want to go, just rest at home.

⑴ 你不想去，就在家休息吧。

Nǐ bùxiǎng qù, jiù zàijiā xiūxí ba.

The coffee here is good. Let’s drink coffee.

⑵ 这儿的咖啡不错，就喝咖啡吧。

Zhè’er de kāfēi bùcuò, jiù hē kāfēi ba.

Today is Dad’s birthday. Let’s go out for dinner.

⑶ 今天是爸爸的生日，我们就去外面吃饭吧。

Jīntiān shì bàba de shēngrì, wǒmen jiù qù wàimiàn chīfàn ba.

Welcome to have my 1-ON-1 lessons on http://www.verbling.com/teachers/dawei 🙂