Learn Hardcore Russian: Сегодня я бегаю медленнее, чем вчера.

бегать – multidirectional / habitual: to run in general, to run back and forth, to run regularly.
- 1st person singular: я бегаю
бежать – unidirectional: to run in one specific direction, usually one concrete movement that is happening now.
- 1st person singular: я бегу

In Сегодня я бегаю медленнее, чем вчера, бегаю suggests a general or repeated action today (e.g. “my runs today are slower than yesterday’s”).

If you wanted to focus on one specific run that is in progress right now, you would more likely say:

Сегодня я бегу медленнее, чем вчера. – Right now I’m running more slowly than I did yesterday (in a similar situation).

Does я бегаю mean “I run” or “I am running”? How should I think of this tense?

Russian doesn’t have a separate present continuous form like English. The same present form covers both:

я бегаю can mean:
- I run (habitually / regularly)
- I am running (right now), if the context makes that clear

In this sentence, Сегодня я бегаю медленнее, чем вчера, context with сегодня (“today”) often suggests something like:

“Today (in general / during today’s training) I’m running more slowly than yesterday.”

If you really want to stress that it’s right now, in this very moment, бегу is usually more natural: Сейчас я бегу медленнее, чем вчера.

What exactly is медленнее? Is it an adjective or an adverb here?

Медленнее is the comparative form of медленно (“slowly”), so here it functions as an adverb: “more slowly”.

Formation:

медленный – slow (adjective)
медленно – slowly (adverb)
медленнее – more slowly / slower (comparative form, used for both adj. and adv. contexts)

In this sentence it modifies the verb бегаю (how I run), so grammatically it’s an adverbial comparative: I run more slowly.

Why is it медленнее and not более медленно?

Both are grammatically possible, but they’re not equally natural:

медленнее – synthetic comparative (one word): the normal, neutral way.
более медленно – analytic comparative (“more slowly” literally): grammatically correct but sounds more formal or awkward here.

Russian strongly prefers the one‑word comparative (быстрее, лучше, холоднее, медленнее) in everyday speech.

So Сегодня я бегаю медленнее, чем вчера is much more natural than Сегодня я бегаю более медленно, чем вчера.

Can I say медленней instead of медленнее? Is that wrong?

Медленнее and медленней are both understood and both exist:

медленнее – the standard literary form; preferred in writing, textbooks, and formal speech.
медленней – a shortened variant; common in colloquial speech and also considered acceptable.

In careful, written Russian (essays, exams, textbooks), stick to медленнее.
In casual spoken Russian, медленней will also sound natural.

Why do we use чем here? Is it always used for “than” in comparisons?

In this kind of comparison, чем functions very much like English than:

медленнее, чем вчера – more slowly than yesterday
быстрее, чем раньше – faster than before

Typical pattern:

[comparative] + чем + [thing being compared]

So:

Сегодня я бегаю медленнее, чем вчера.
Он выше, чем его брат. – He is taller than his brother.

Sometimes Russian can use the genitive instead of чем (e.g. Он выше брата), but you can’t say *медленнее вчера in standard Russian. With adverbial time words like вчера, сегодня, you need чем.

Why is there a comma before чем?

In Russian punctuation, чем in a comparison like this usually introduces a separate comparative clause or phrase, so it’s normally preceded by a comma:

… медленнее, чем вчера.
… быстрее, чем раньше.

General rule: when чем introduces a comparison after a comparative form (быстрее, лучше, больше, меньше, медленнее, etc.), a comma is usually required.

There are some fixed expressions where the comma can be omitted (e.g. не более чем, не иначе как), but your sentence follows the regular pattern, so the comma is correct and expected.

Can I change the word order, like Я сегодня бегаю медленнее, чем вчера? Is that still correct?

Yes, Russian word order is quite flexible. All of these are grammatically correct:

Сегодня я бегаю медленнее, чем вчера. (neutral)
Я сегодня бегаю медленнее, чем вчера.
Я бегаю сегодня медленнее, чем вчера.

They all mean essentially the same thing. The differences are mostly about emphasis:

Putting сегодня first (your original sentence) highlights today as the topic.
Putting я first sounds more like “As for me, today I run more slowly than yesterday.”
Moving сегодня after бегаю can slightly emphasize the verb бегаю.

In everyday speech, Сегодня я бегаю… and Я сегодня бегаю… are probably the most common patterns.

How would I say “Today I ran more slowly than yesterday” (past, already finished)?

You need the past tense of the verb:

Imperfective past (process / repeated action):
- Сегодня я бегал медленнее, чем вчера.
- “Today I was running / I ran more slowly than yesterday.”
- Often describes the general quality of your running today (e.g. during training).
Perfective past (completed event as a whole):
- Сегодня я пробежал медленнее, чем вчера.
- “Today I completed my run more slowly than yesterday.”
- Focus on one completed run and its result (time, speed).

Choice depends on what you want to emphasize: the overall process (бегал) or a specific completed run (пробежал).

Why don’t сегодня and вчера change form? Are they nouns or something else?

Сегодня (“today”) and вчера (“yesterday”) are adverbs of time, not nouns.

Adverbs in Russian do not decline for case, number, or gender.
That’s why they keep the same form regardless of their role in the sentence.

So you always say:

сегодня – today
вчера – yesterday
завтра – tomorrow

They never become “сегодням”, “сегодняу”, “вчерам”, etc. There are related nouns (сегодняшний день, вчерашний день), but those are different words.

Could I repeat the verb after чем, like … чем вчера бегал? Is that more correct?

You can repeat the verb, but it isn’t necessary:

Сегодня я бегаю медленнее, чем вчера. – Standard, natural.
Сегодня я бегаю медленнее, чем вчера бегал. – Also grammatically correct, but sounds heavier and usually unnecessary in speech.

Russians normally omit the repeated verb if the meaning is clear.
You’d repeat it mainly for emphasis or clarity in more complex sentences. Here, the short version is better.

How would I say “Today I’m running more slowly than I usually do,” not “than yesterday”?

You can express “than I usually do” with обычно (“usually”) or обычно бегаю (“I usually run”):

Сегодня я бегаю медленнее, чем обычно.
– Today I’m running more slowly than (I) usually (do).

If you want to make the verb explicit:

Сегодня я бегаю медленнее, чем я обычно бегаю.

The shorter version (чем обычно) is more natural in everyday speech.