You're talking with a friend about a book that you read recently. You really liked it, and you think your friend should read it to. You say:
I highly recommend it.
To suggest that a person should do something, say "I recommend ___". For example:
I recommend the cheese fries. They're amazing!
I recommend hiring a lawyer early on in the process, to make sure that you don't miss anything important.
As you can see, you can either say "I recommend (noun)" or "I recommend (doing something)".
When you use this phrase, it just sounds like a simple suggestion. It doesn't sound especially bossy, polite, helpful, or rude.
It's common to include an adverb before "recommend". The most common ones are:
Doctors generally recommend keeping the sprain elevated, applying ice packs, and wrapping the area up tightly.
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