С тех пор как я поставил будильник на семь утра, я больше не опаздываю на работу.

It means since or ever since, literally something like from the time when.

So:

С тех пор как я поставил будильник на семь утра...
= Since I set my alarm for seven in the morning...

It introduces the starting point for the situation in the main clause.

After the preposition с in this expression, Russian uses the genitive:

• те → тех
• поры / пора in this fixed expression appears as пор

So с тех пор literally means from those times / from that point onward.

You do not need to build this phrase word by word in normal speech; it is best learned as a fixed expression: с тех пор как = since.

Because the sentence refers to a completed action that happened earlier: at some point in the past, the speaker set the alarm.

Russian often uses the past tense after с тех пор как when talking about the event that started a later situation:

• я поставил будильник = I set the alarm

That past action becomes the starting point for what happens now.

Поставил is perfective, so it presents the action as completed: the alarm was set.

That fits the meaning well here: one completed action led to a continuing result.

• поставил = set / finished setting
• ставил would suggest a process, repetition, or an incomplete/background action

So с тех пор как я поставил будильник... means ever since I set the alarm..., not while I was setting the alarm or used to set the alarm.

Because the main clause describes the speaker’s current ongoing situation:

я больше не опаздываю на работу
= I’m no longer late for work / I don’t arrive late for work anymore

Russian commonly uses the present tense here because the result still holds now. The idea is:

• a past event happened: я поставил будильник
• since then, the present situation has changed: я больше не опаздываю

Because опаздывать is the imperfective verb, and here it describes a repeated or habitual situation: being late to work in general.

• опаздываю = I am late / I tend to be late / I keep arriving late
• опоздаю = I will be late once or I’ll arrive late

The sentence is not about one future instance. It is about a repeated pattern that no longer happens. That is why опаздываю is the natural choice.

It means to set an alarm.

Even though будильник literally means alarm clock, in modern Russian this phrase is also used for setting the alarm on a phone.

So:

• поставить будильник = set an alarm
• not usually put an alarm clock somewhere

In this sentence, it means the speaker set the alarm to go off at a certain time.

Because Russian uses поставить будильник на + time to mean set an alarm for + time.

So:

• на семь утра = for seven in the morning

This is a standard pattern:

• поставить будильник на шесть
• поставить будильник на восемь тридцать
• поставить будильник на завтра

Here на is the normal preposition used with the time the alarm is set for.

Because Russian often drops часов when the time is already clear.

So these are both possible:

• на семь утра
• на семь часов утра

The shorter version is very common and natural.

Also, утра helps show that this is 7 a.m., not 7 p.m.

Больше не means no longer, not anymore.

So:

• я не опаздываю = I am not late
• я больше не опаздываю = I am no longer late / I’m not late anymore

The word больше adds the idea that this used to happen before, but now it has stopped. That matches the meaning of the whole sentence very well.

Because Russian says опаздывать на + accusative for many destinations, events, or obligations that someone is late for:

• опаздывать на работу = be late for work
• опаздывать на поезд = be late for the train
• опаздывать на урок = be late for class
• опаздывать на встречу = be late for a meeting

So на работу is just the normal pattern after опаздывать here.

Because С тех пор как я поставил будильник на семь утра is a subordinate clause, and я больше не опаздываю на работу is the main clause.

Russian separates them with a comma:

С тех пор как я поставил будильник на семь утра, я больше не опаздываю на работу.

Notice that the comma is placed between the clauses, not normally inside the fixed expression с тех пор как.

Yes. Russian word order is flexible. You could also say:

Я больше не опаздываю на работу с тех пор, как поставил будильник на семь утра.

That still means the same thing: I haven’t been late for work since I set the alarm for 7 a.m.

The original version puts the since-clause first, which sounds very natural and gives the background before the main point.