[tlhIngan Hol] What exactly does the {'ej} connect ?

[mayqel qunenoS]

We know that the {'ej} is used, in order to join sentences.

Now, at the risk of sounding like the village idiot, I will ask:

What exactly is a sentence ?

If I write:

"while I was walking, I stepped on a cat", then that's a sentence.

If I write:

"while I was walking and while I was eating, I stepped on a cat", then
that's a sentence too.

But what exactly are the "while I was walking" and "while I was
eating" on their own ?  Are they sentences ?

The reason I'm writing this is because I want to ask:

Can we write:

{jIyIttaHvIS 'ej jISoptaHvIS, vIghro' vIgho'} ?

Or if the {jIyIttaHvIS} and {jISoptaHvIS} aren't sentences on their
own, one cannot join them by the use of {'ej} ?

qunnoH jan puqloD