kesa ha nebousite, mousukosi de sihatu ni noriokureru tokoro datta.

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:

• This morning, ...
• As for this morning, ...

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

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:

• I overslept, and as a result...
• Because I overslept...

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

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:

• もう少しで転ぶところだった。
  I almost fell.
• もう少しで電車に乗り遅れるところだった。
  I almost missed the train.

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

始発 means the first departure of the day, usually the first train, bus, etc.

In this sentence, it most naturally means:

• the first train of the day

Japanese often leaves out words that are obvious from context, so 始発 by itself is enough without saying 電車.

The verb 乗り遅れる means to be late for / to miss (a train, bus, plane, etc.)

With this verb, the thing missed is commonly marked by に:

• 電車に乗り遅れる
• バスに乗り遅れる
• 始発に乗り遅れる

This may feel strange to English speakers because English uses a direct object: miss the train. But in Japanese, this verb naturally takes に.

It helps to think of 乗る first:

• 電車に乗る = to get on the train

Then 乗り遅れる is built from that idea:

• to be too late to get on it

So に stays natural here.

乗り遅れる is made from:

• 乗る = to ride / get on
• 遅れる = to be late

Together, 乗り遅れる means:

• to miss a train/bus/etc. by being late
• literally, something like to be late to board

So it is not a general word for miss in every sense. It is specifically used for failing to catch transportation because you were late.

This is a very common point of confusion.

Verb dictionary form + ところだった means:

• was just about to ...
• almost ...
• was on the verge of ...

So:

• 乗り遅れるところだった = I was about to miss it / I almost missed it

The dictionary form shows that the action had not actually happened.

Compare:

• 乗り遅れた = I missed it
• 乗り遅れるところだった = I almost missed it, but didn’t

That is why the non-past form 乗り遅れる is used before ところだった.

ところ literally means place, but in grammar it often means something like point, moment, or stage.

In V-る + ところだった, it expresses that the speaker was at the point right before something happened.

So:

• 乗り遅れるところだった

means:

• I was at the point of missing it
• I almost missed it

This pattern is very common:

• 死ぬところだった = I almost died
• 忘れるところだった = I almost forgot
• 遅刻するところだった = I almost was late

The だった is past because the whole situation is being described as something that happened earlier.

So even though 乗り遅れる is in dictionary form, the full pattern 乗り遅れるところだった means:

• was about to miss
• almost missed

The important idea is:

• 乗り遅れる = the action that was about to happen
• ところだった = this was the situation in the past

So the sentence is talking about a past near-event.

Yes. Japanese often leaves out the subject when it is obvious from context.

Here, the implied subject is probably I:

• (私は) 今朝は寝坊して、もう少しで始発に乗り遅れるところだった。

In English, we usually need to say I, but in Japanese it is very often omitted if the listener can easily understand who is being talked about.

Yes. The sentence has a natural cause → near consequence flow:

• 寝坊して = I overslept
• もう少しで始発に乗り遅れるところだった = I almost missed the first train

So the speaker is not just listing two things. The first part explains why the second part nearly happened.

It gives a very natural storytelling feel:

• This morning, I overslept, and I almost missed the first train.

Yes, but the nuance changes a little.

• 始発に乗り遅れるところだった already means I almost missed the first train
• Adding もう少しで makes the almost feeling stronger and more explicit

So with もう少しで, it sounds more like:

• I very nearly missed it
• I was this close to missing it

It emphasizes how close the speaker was to that bad outcome.