现在买这个手机不一定是最好的时候。

Xiànzài mǎi zhège shǒujī bú yídìng shì zuìhǎo de shíhou.

Buying this phone now is not necessarily the best time.

努力学习的人不一定马上成功，但会越来越好。

Nǔlì xuéxí de rén bù yídìng mǎshàng chénggōng, dàn huì yuèláiyuè hǎo.

People who study hard may not succeed right away, but they will get better and better.

今天不一定下雨，你还是带把伞吧。

Jīntiān bù yídìng xiàyǔ, nǐ háishi dài bǎ sǎn ba.

It may not rain today, but you’d better take an umbrella.