Na záhonu pleju plevel.

Because na záhonu expresses a location: on/in the flower bed.

With na, Czech uses:

• locative for location: na záhonu = on/in the flower bed
• accusative for motion toward a place: na záhon = onto/to the flower bed

So:

• Jsem na záhonu. = I am in/on the flower bed.
• Jdu na záhon. = I am going to the flower bed.

In your sentence, the speaker is already there, doing the weeding, so Czech uses the locative: na záhonu.

It is locative singular of záhon.

The basic form is:

• záhon = flower bed / garden bed

After the preposition na when it means location, the noun goes into the locative:

• na záhonu

This is a very common pattern in Czech:

• na stole = on the table
• na zahradě = in the garden
• na záhonu = in/on the flower bed

Because Czech and English do not always divide space in the same way.

Literally, na often corresponds to on, but with some places and surfaces it is used where English prefers in or at. A flower bed is thought of as a kind of defined gardening area, so na záhonu is the normal Czech expression.

So even though the literal translation may look like on the flower bed, the natural English meaning is usually in the flower bed or in the garden bed.

This is something you often just have to learn as a collocation:

• na zahradě = in the garden
• na poli = in the field
• na záhonu = in the flower bed

Pleju is the 1st person singular present tense: I weed.

It comes from the verb plít = to weed.

So:

• pleju = I weed / I am weeding
• pleješ = you weed
• pleje = he/she/it weeds

Czech present tense can often correspond to either:

• I weed
• I am weeding

The exact English translation depends on context.

Because plít is an irregular or at least less predictable verb in the present tense.

Its present stem changes:

• infinitive: plít
• present: plej-

So:

• pleju
• pleješ
• pleje
• plejeme
• plejete
• plejí

This kind of stem change is something learners usually just memorize. Czech has a number of verbs whose present tense is not built in a simple, transparent way from the infinitive.

Plevel is the direct object, so it is in the accusative.

The noun is:

• plevel = weed / weeds

Here, plevel is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: plevel
• accusative: plevel

That is why the form does not visibly change in this sentence.

Usually plevel is understood as a collective/general noun meaning weeds or weed growth, not necessarily one single weed.

So in this sentence, pleju plevel is best understood as:

• I am weeding out weeds
• I’m pulling weeds

Even though the form is singular, the meaning is often more general or mass-like.

It is understandable, but it can sound a bit repetitive or textbook-like, because pleju plevel is very close to I weed weeds.

In real life, a Czech speaker might more naturally say:

• Pleju záhon. = I’m weeding the flower bed.
• Pleju na záhonu. = I’m weeding in the flower bed.
• Vytrhávám plevel na záhonu. = I’m pulling weeds in the flower bed.

So your sentence is grammatically fine and useful for learning, but not necessarily the most idiomatic everyday phrasing.

Czech word order is much more flexible than English word order.

Both of these are possible:

• Na záhonu pleju plevel.
• Pleju plevel na záhonu.

The difference is mainly one of focus or emphasis.

Starting with Na záhonu puts the location first, as if answering:

• Where are you weeding?
  Na záhonu pleju plevel.

Starting with Pleju puts the action first:

• What are you doing?
  Pleju plevel na záhonu.

So the original sentence is not strange; it just highlights the location first.

Yes. You can say simply:

• Pleju plevel. = I’m weeding / I’m pulling weeds.

Whether you include na záhonu depends on whether the location matters.

If the place is obvious from context, Czech often leaves it out. If you want to specify where the action happens, then na záhonu is useful.

A simple approximate pronunciation is:

na ZAA-ho-noo PLEH-yoo PLEH-vel

More carefully:

• Na = na
• záhonu = zaa-ho-nu, with stress on the first syllable
• pleju = roughly pleh-yoo
• plevel = pleh-vel

A few useful pronunciation notes:

• Czech stress is usually on the first syllable of each word.
• á is a long vowel.
• In pleju, the j sounds like English y in yes.

Plít is imperfective.

That matters because the sentence describes an ongoing or habitual activity:

• I am weeding
• I weed

Imperfective verbs are the normal choice for:

• ongoing actions
• repeated actions
• general activities

So pleju plevel sounds like a normal description of the activity itself, not a completed one-time result.

It can mean a garden bed more generally, not only a flower bed.

Depending on context, záhon may refer to:

• a flower bed
• a vegetable bed
• a planted garden strip/bed

So na záhonu is broader than just a decorative flower bed. The exact English translation depends on what is being grown there.