syntax - How to know if a sentence ending in the て form of a verb is imperative or not

I know that a sentence that ends in the て form of a verb can be imperative. How do you know for sure if it is? Why is「～しようとして」not imperative?

Answer

As sawa said, the て form has many usages. One of them being used to formulate requests (what you're confusing with the grammatical term "imperative")

しようとして (and all the forms in ようとして：食べようとして...etc.) can all be perfectly correct requests or commands depending on how the sentence ends.

To understand this, you need to know that the よう form of a verb is called Volitional and is used (in brief) to say "let's do..."

Now, adding として to this volitional form, it should be decomposed as follows:

-と is the particle (same as と言う、と思う)

-して (て form of する)

• If the sentence doesn't end after the て form:

虫が草を食べようとして、鳥に食べられた。

The insect was about to eat grass when he got eaten by a bird.

Then the て form is used to combine sentences (see the Wikipedia article above). ようとして then means "to be about to" or "try to".

• On the contrary, if the sentence ends with として, then you have the request form.

The volitional form of a verb + として corresponds to the English "(Please) Try to...". (you can still add ください、ほしい、くれ...etc. like for any other request) It's a very natural way to request someone to do something and see how it goes.

Some widely used examples to illustrate this:

(browser doesn't display flash) フラッシュをダウンロードしようとしてください。Please try to download flash.

(internet connection lost) 再度接続しようとしてください。Please try to reconnect.

このポイントを明らかにしようとしてください。Please try to clarify this point.