なるべく
as much as possible, if possible
なるべく + Verb / adjective
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なるべく means "as much as possible" or "where possible": なるべく早く来てください, "please come as early as you can".
It's softer than an absolute rule. It asks for effort within reason: なるべく野菜を食べるようにしている, "I try to eat vegetables where I can". With a negative, it's "avoid if you can": 夜はなるべく外に出ないでください.
It's interchangeable with できるだけ in almost every sentence.
Compare 必ず ("without fail") and 絶対に ("absolutely"), which are strict. なるべく leaves room: do your best, but it's not a rule. In polite requests, なるべく softens what you're asking, so it's very common at work.
Examples
なるべく早く来てください。
Please come as early as you can.
なるべく野菜を食べるようにしている。
I try to eat vegetables as much as possible.
夜はなるべく外に出ないでください。
Please avoid going out at night as far as possible.
荷物はなるべく少なくしたほうがいい。
You'd better keep your luggage as light as you can.
なるべく日本語で話しましょう。
Let's speak in Japanese as much as we can.