See it happen or see it happening?

A Introduction

David fell down the steps. Rachel saw him fall.

David was walking with a stick. Rachel saw him walking across the road.

After some verbs we can use an object + infinitive without to.

VERB OBJECT INFINITIVE

Rachel saw David fall down the steps.

Vicky heard someone close the door.

Let's watch the parade go past.

We all felt the house shake.

We can use this structure with these verbs: feel, hear, listen to, notice, see, watch

We can also use an -ing form after the object.

VERB OBJECT -ING FORM

Rachel saw David walking with a stick.

Can you hear someone playing the piano?

We found Matthew exercising in the gym.

I could feel an insect crawling up my leg.

We can use this structure with these verbs: feel, find, hear, listen to, notice, see, smell, watch

SEE IT HAPPEN

We saw Trevor plant the tree. (He planted the tree. We saw him do the whole job.)

I watched Nick light a cigarette.

We noticed a young man sit down and order a meal.

SEE IT HAPPENING

We saw Trevor planting the tree. (He was planting the tree. We saw him in the middle of the job.)

I watched Nick smoking a cigarette.

We noticed a young man sitting at the table eating a meal.

When we talk about a short action, it often does not matter which structure we use.

They heard a car turn/turning the corner.

I didn't see anyone leave/leaving any litter.