1. Explain it simply
来得及 means there is still enough time to do something before a deadline.
来不及 is the opposite: there is not enough time, it is too late.
They are fixed words, so keep each one whole: S + (还) 来得及 / 来不及 + verb phrase.
还 often comes before 来得及 to stress that there is still time: 现在出发还来得及.
2. Show examples
现在出发还来得及。
Xiànzài chūfā hái láidejí.
There is still time to leave now.
还来得及 = there is still time
太晚了，已经来不及了。
Tài wǎn le, yǐjīng láibùjí le.
It is too late; there is no longer enough time.
来不及 = too late now
现在走还来得及。
Xiànzài zǒu hái láidejí.
There is still time to go now.
fixed word, kept whole
3. Share a tip
来得及 and 来不及 are fixed words - keep them whole and after the subject: say 现在还来得及, not 来得及现在还。