Learn Hardcore French: Le dimanche matin, je fais de la course à pied dans le parc avec une amie. - On Sunday morning, I go running in the park with a friend.

Le dimanche matin = on Sunday mornings / every Sunday morning (a regular habit).
Dimanche matin (without le) is more like on Sunday morning (one specific Sunday, usually understood from context).

So le + day of the week expresses a repeated, habitual action.

Why is dimanche matin singular if it means “on Sunday mornings”? Why not les dimanches matins?

French normally keeps the day and the time expression in the singular after le to talk about a repeated habit:

Le dimanche matin, je fais… = On Sunday mornings, I…
Le lundi soir, je regarde la télé. = On Monday evenings, I watch TV.

You could say tous les dimanches matin, but the simple le dimanche matin already carries the idea of repetition, so plural is not needed.

Could I also say Je fais de la course à pied le dimanche matin instead?

Yes. Both are correct:

Le dimanche matin, je fais de la course à pied…
Je fais de la course à pied le dimanche matin…

French often puts time expressions either:

at the beginning (for emphasis on when), or
at the end (neutral placement).

Meaning doesn’t change; it’s just a question of style and emphasis.

Why do we say je fais de la course à pied instead of just je cours?

Both are possible, but they’re used a bit differently:

Je cours dans le parc. = I run in the park.
Simple verb, neutral, very common.
Je fais de la course à pied. = literally I do some running (as a sport).
It presents running more as a sport / activity you practise.

In French, for many sports and hobbies we often use faire + de la / du / des + activity:

faire du vélo – to go cycling
faire du ski – to go skiing
faire de la natation – to do swimming

So faire de la course à pied fits this pattern of “doing the sport of running.”

What exactly does course à pied mean?

Literally, course à pied means “running on foot”.

course = race / running
à pied = on foot

Together, course à pied is the general term for the sport of running / jogging (as opposed to cycling, driving, etc.). In everyday speech, people often say:

Je cours. – I run.
Je fais du jogging. – I go jogging. (informal/borrowed from English)

All are understandable, but faire de la course à pied is a neutral, standard way to say you practise running as a sport.

Why is it de la course à pied and not de le course à pied or something else?

Course is a feminine noun, so the correct partitive article is de la:

de le does not exist; it contracts to du, and is used for masculine singular nouns:
du tennis, du foot, du ski
de la is used for feminine singular nouns:
de la natation, de la danse, de la course à pied

So faire de la course à pied literally means “to do some running” (the partitive de la is like “some” in English in this context).

What is the difference between dans le parc and au parc?

Both can be used, but the nuance is:

dans le parc = inside the park, within its boundaries.
Focus on the physical space you are in.
au parc = at/to the park (from à + le = au).
Focus more on the location as a place where the activity happens.

In many everyday contexts, both are acceptable and will sound natural:

Je cours dans le parc.
Je cours au parc.

Here, dans le parc emphasizes that your running takes place inside the park itself.

Why is it avec une amie and not avec un ami?

The noun amie with an -e is the feminine form of ami (“friend”).
So:

un ami = a (male) friend
une amie = a (female) friend

In the sentence, une amie tells us the friend you run with is female.

Does amie sound different from ami when spoken?

In standard French pronunciation, un ami and une amie are pronounced the same:

ami / amie → /a.mi/

The difference between ami and amie is only in writing, not in pronunciation.
We hear the gender from the article (un vs une) and/or from context.

Why is it avec une amie and not avec mon amie if it’s “with a friend of mine”?

Une amie is an indefinite expression: a friend (of mine), not specified which one.
Mon amie is definite: my friend (one specific person that the listener can identify from context).

Je cours avec une amie.
I run with a (female) friend (no need for the listener to know who).
Je cours avec mon amie.
I run with my (female) friend (the person is known or already mentioned).

Also note: mon amie can sometimes suggest a romantic partner (“my girlfriend”), depending on context, so une amie is often safer if you just mean “a female friend.”

Why is the present tense je fais used here for a habitual action?

In French, the present tense is used both for:

Actions happening right now
- Je fais la vaisselle. – I’m doing the dishes (now).
Habits or repeated actions
- Je fais de la course à pied le dimanche matin. – I (regularly) go running on Sunday mornings.

So je fais here is the normal way to say “I (usually) go running” or “I run” on Sundays. No special tense is needed.