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

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