Learn Hardcore Japanese: kesa ha nebousite, mousukosi de sihatu ni noriokureru tokoro datta. - This morning I overslept and almost missed the first train.

Breakdown of kesa wa neboushite, mousukoshi de shihatsu ni noriokureru tokoro datta.

Why is 今朝 followed by は here?

は marks 今朝 as the topic: as for this morning or this morning.

With time words, Japanese often uses は when the speaker is setting the scene or contrasting that time with others. So 今朝は sounds like:

The sentence would still be understandable without は in some contexts, but 今朝は is very natural when introducing what happened this morning.

What does 寝坊して mean, and why is it in the -て form?

寝坊する means to oversleep or to sleep in too much.

So 寝坊して is the -て form of that verb. Here, the -て form connects this action to what follows:

In this sentence, it gives the reason/background for the near-miss that comes next.

What does もう少しで mean in this sentence?

もう少しで means almost, just about, or literally with just a little more...

It is often used when something was very close to happening, especially something undesirable or surprising.

Examples:

So here it sets up the idea that the speaker came very close to missing the first train.