I found in this online dictionary http://www.reference.com/example-sentences/vigilant?s=1 a sample sentence of the form

We need to be more vigilant concerning about our environment, let keep it clean and let reduce our emission.

But the use of let keep it clean transcends my understanding of the word let. Always I use let in a sentence such as Let something be something. So I wish to know the general rule behind this usage.

2 Answers 2


Those are typos, and there's a grammatical error in the sentence as well. It should read thus:

We need to be more vigilant about our environment. Let us keep it clean and let us reduce our emissions.

You could also use "concerning" instead of "about", but not both. You could also use "let's" instead of "let us"; "let us" is quite formal but I wanted to show the construction more clearly. Also, the sentences are run together and should be separated. If you do want to run them together, you should use a semicolon rather than a comma.

So, your understanding of the word let is correct, and the understanding of whoever wrote the sentence is incorrect. I will add that the general rule behind this usage is this: the first person plural imperative is constructed by using "let us" in front of the verb. For example, "Let's go to Joe's" is a suggestion that we go to Joe's house together. It's similar to "Allons chez Joe's" in French, or "Gehen wir nach Joe's" in German.

  • Thanks very much. Generally I use Let something be something for mathematical convenience (and indeed a convention, simply). For instance, we usually write, say Let f be a function or several things alike.
    – Yes
    Jul 13, 2014 at 3:25
  • Yes, most certainly that is a use of the word let. Let generally has the meaning of allow, for example "let me take care of that". A few idiomatic uses (and there are plenty): "Let me be" means leave me alone, "Let him know" means inform him, "let him off" means forgo some sort of punishment, "let me down" means disappoint me (cf. the Beatles song "Don't Let Me Down"), "let him down easily" means be gentle when you inform him of something disappointing.
    – BobRodes
    Jul 13, 2014 at 3:34
  • Much appreciated for it; that is comprehensive.
    – Yes
    Jul 13, 2014 at 4:19

The grammar in that sentence is terrible. It should read:

We need to be more vigilant concerning our environment, let's keep it clean and let's reduce our emission.

let's (a contraction of let + us) is used to form plural first-person commands in English. For instance,

Let's go!

This is a command, it is demanding that me and whomever I am speaking with leave now.

This can be used in softer contexts as well:

Let's see a movie tonight.

This is still a command, but it appears almost as a suggestion.

So in the sentence "let's keep it clean", the speaker is heavily suggesting that they as well as the rest of the world need to keep it clean (maintain cleanliness).

  • 1
    Great minds thinking alike. :)
    – BobRodes
    Jul 13, 2014 at 3:14

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