趁: while, taking advantage of (趁着, 趁早, 趁机)
【高51】介词 / prepositions (凭借、依据)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
趁 introduces a favourable but passing circumstance, and the main clause says what one should or can do while it lasts: “taking advantage of, while, as long as”. 趁你还年轻，一定要多尝试; 面条要趁热吃.
The condition must be still true and about to pass, hence the sense of advice, urgency or opportunity. The clause often contains 还 or 没 (趁天还没黑, 趁火车还没开). You also meet 趁着 (same meaning, more rhythmic) and set uses: 趁热 (while it's hot), 趁早 (as early as possible, before it is too late), 趁机 (to seize the chance, often in a bad sense: 趁机散布谣言) and 趁火打劫 (to profit from someone's misfortune).
Difference from 的时候: 的时候 merely dates an event; 趁 adds that someone makes use of it.
Examples
面条要趁热吃。
Miàntiáo yào chèn rè chī.Noodles should be eaten while they're hot.
趁天还没黑，我们快点回去吧。
Chèn tiān hái méi hēi, wǒmen kuài diǎn huíqu ba.Let's hurry back while it's still light.
趁你还年轻，一定要多尝试。
Chèn nǐ hái niánqīng, yídìng yào duō chángshì.While you're still young, be sure to try lots of things.
这个问题要趁早解决，拖得越久越麻烦。
Zhège wèntí yào chènzǎo jiějué, tuō de yuè jiǔ yuè máfan.This problem should be solved as early as possible; the longer you put it off, the more trouble it becomes.
他趁大家不注意，悄悄溜了出去。
Tā chèn dàjiā bú zhùyì, qiāoqiāo liū le chūqu.While nobody was paying attention, he slipped quietly out.
趁着周末，我们全家去山里住了两天。
Chènzhe zhōumò, wǒmen quánjiā qù shān li zhù le liǎng tiān.Taking advantage of the weekend, the whole family spent two days in the mountains.
Common mistakes
✗ 趁我吃饭的时候，他打电话来了。 → ✓ 我吃饭的时候，他打电话来了。 (nobody is exploiting the moment; for a mere coincidence use 的时候).
✗ 趁现在没有机会，你去吧。 → ✓ 趁现在还有机会，你去吧。 (the circumstance must be favourable and still true).