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Could you please tell me which sentence correctly uses "to"? I am always a little confused when using "to" in a sentence.

Here, if it is a verb then "to" is required.

This will help us to ensure that

or

This will help us ensure that

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This will help us [to ensure that].

With the verb help, it is okay either to use or not to use to. The part in brackets is called an infinitival clause, and when we don't use to, it's called a bare infinitival:

This will help us [ensure that.]

The verb help is called a catenative verb: it can be "connected" to another verb ("ensure").

There is a tendency with some speakers to drop to when help is more direct, and to retain to when help is less direct:

She helped me (to) prepare my homework by disassembling my computer, so that I had nothing to do but study. (the help is indirect)


There are not a lot of catenative verbs that always take "bare infinitivals":

  1. Modal verbs (will, shall, can..) and supportive do:

I can ensure that. He did ensure that.

  1. Sensory perception verbs (feel, hear, notice, see, watch, observe, overhear)

I saw him steal the car.
I felt my heart sink.

  1. Some of the causative verbs: have, let, make:

I let them play in my backyard.

His silly poem made her smile.

And there are several verbs that can take either a "bare infinitival" or a "to-infinitival": ought, dare, (had/have) known, help:

I had never known him (to) lose his temper before he fell ill. (know can take the bare infinitival only is this specific sense and when it is in the Perfect Tense)

Audacious tyrant, do you dare to beard a maiden in her lair! How dare you challenge me! (with and without "to")


And one additional point:

When the matrix ("main") clause is in the Passive Voice, we use to, even with the catenative verbs I've just mentioned:

He was made to swear allegiance to the King.

He was heard to cry out in protest. "No. Not in the Bible. With respect, Your Grace, I ask to speak on this absolute impediment, please."

The player was seen to leave the field.


Reference:

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