Neměl bys běhat moc rychle.

In this sentence, neměl bys means you shouldn’t.

More literally, it is built from mít and a conditional form, but in practice měl bys + infinitive is a very common way to say you should + verb:

• Měl bys běhat. = You should run.
• Neměl bys běhat. = You shouldn’t run.

So here it expresses advice, not a hard command.

That is a very common point of confusion. Měl does look like the past form of mít, but here the whole construction is conditional, not past tense.

Czech forms the conditional with:

• an l-participle such as měl
• plus by / bys / bychom / byste etc.

So měl bys does not mean you had here. It means something like you should / you would be supposed to.

Bys is the 2nd person singular conditional particle, used with ty.

Very roughly, it carries the you would part of the meaning.

So:

• měl bych = I should / I would have
• měl bys = you should
• měl by = he should

Historically it comes from by jsi, and learners may sometimes see both, but bys is the normal compact form.

Because Czech marks gender in this kind of form.

• Neměl bys... is addressed to one male
• Neměla bys... is addressed to one female

So if you were speaking to a woman, the sentence would be:

Neměla bys běhat moc rychle.

Because Czech often leaves subject pronouns out when they are clear from the verb form.

In English, you usually need you. In Czech, bys already shows that the sentence is addressed to you singular, so ty is unnecessary.

You could add ty for emphasis or contrast, but it is not needed in a neutral sentence.

Běhat means to run in a more general or repeated sense: running as an activity, running around, running regularly.

That makes it natural in advice like this.

A learner may wonder about běžet. Very roughly:

• běhat = running as an activity in general / repeatedly
• běžet = to be running, or one more specific running event

So Neměl bys běhat moc rychle sounds like general advice about how you run. With běžet, it would sound more tied to a specific occasion or moment.

Because rychle is an adverb, and it describes how someone runs.

• běhat rychle = to run fast

By contrast, rychlý is an adjective, and adjectives describe nouns:

• rychlý pes = a fast dog

Since the word is modifying the verb běhat, Czech needs the adverb rychle.

Here moc means too much / too, so:

• moc rychle = too fast

That is why the sentence is understood as You shouldn’t run too fast.

Be careful, though: moc can mean different things in different contexts, such as a lot, very, or too much. In this sentence, too is the best match.

Because in Czech, when you have a should-type construction, the negation usually goes on the finite part:

• Neměl bys běhat... = You shouldn’t run...

This is the normal, neutral way to say it.

If you said Měl bys neběhat..., that would sound much less neutral and more like You should refrain from running... or You should make a point of not running... The usual sentence is therefore neměl bys běhat.

Yes, to some extent.

The neutral order is:

Neměl bys běhat moc rychle.

But you can also hear:

Neměl bys moc rychle běhat.

The meaning stays basically the same. The difference is mostly about emphasis and rhythm.

One important thing, though: bys is a clitic, so it normally stays near the beginning of the sentence, in the so-called second position.

For plural or formal you, Czech uses byste and plural agreement:

Neměli byste běhat moc rychle.

So the main patterns are:

• Neměl bys... = to one male
• Neměla bys... = to one female
• Neměli byste... = to several people or formal you

No. Neměl bys... is softer.

• Neměl bys běhat moc rychle. = You shouldn’t run too fast.
  This sounds like advice or a recommendation.

• Nesmíš běhat moc rychle. = You must not run too fast.
  This sounds much stronger: prohibition, rule, or serious warning.

So neměl bys is often the better choice when you are giving advice rather than forbidding something.