1. Explain it simply
This 就 means something happens as early or as quickly as that.
Put 就 immediately before the verb, usually after a time or amount phrase.
The pattern is [time/amount] + 就 + verb, as in 二十分钟就到了.
2. Show examples
二十分钟就到了。
Èrshí fēnzhōng jiù dào le.
It only took twenty minutes to get there.
amount + 就 + verb
他七点就来了。
Tā qī diǎn jiù lái le.
He came as early as seven o'clock.
time + 就 + verb
坐飞机两个小时就到北京了。
Zuò fēijī liǎng gè xiǎoshí jiù dào Běijīng le.
By plane it's only two hours to Beijing.
就 = in just
3. Share a tip
Keep 就 directly in front of the verb - never after the verb, at the end of the sentence, or before the subject.