Продавець поклав пучок кропу на ваги.

Because after пучок (a bunch), Ukrainian normally uses the genitive case for the thing being measured or grouped.

So:

• кріп = dill (dictionary form, nominative)
• пучок кропу = a bunch of dill

This is similar to other patterns like:

• склянка води = a glass of water
• кілограм яблук = a kilogram of apples

So кропу is the genitive singular form of кріп.

It is functioning as the direct object, so it is in the accusative case. But for many inanimate masculine nouns in Ukrainian, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: пучок
• accusative: пучок

That is why the form does not visibly change.

Because this sentence describes movement onto the scales, not location on the scales.

With на:

• на + accusative = motion onto
• на + locative = location on

So:

• поклав на ваги = put onto the scales
• лежить на вагах = is lying on the scales

Here the seller moved the dill onto the scales, so ваги is in the accusative plural.

In this meaning, ваги (scales) is usually used as a plural-only noun. Ukrainian treats weighing scales like an item with two parts, somewhat like how English often uses scales rather than a scale in everyday shop contexts.

So learners should get used to:

• ваги = scales
• на вагах = on the scales
• покласти на ваги = to place on the scales

Поклав is the past tense, masculine singular form of the verb покласти.

It tells us that:

• the action happened in the past
• the subject is masculine
• the verb is perfective, so it presents the action as completed

So поклав means something like put/placed in the sense of a finished action.

Because the subject is продавець (seller, masculine), the verb is masculine too.

This is a question of aspect.

• класти / клав = imperfective
  Focuses on process, repetition, or ongoing action.
• покласти / поклав = perfective
  Focuses on a single completed action.

In this sentence, the seller completed the action of placing the dill on the scales, so поклав is the natural choice.

Very roughly:

• він клав кріп на ваги = he was putting / used to put dill on the scales
• він поклав кріп на ваги = he put the dill on the scales

In this sentence, продавець is grammatically masculine, and that is why the verb is поклав.

If you wanted to refer clearly to a female seller, you could use a feminine noun and feminine verb form, for example:

• Продавчиня поклала пучок кропу на ваги.

So the sentence as written describes a male seller, or at least uses a masculine noun.

Yes. Ukrainian word order is fairly flexible because the cases show the grammatical roles.

The neutral order here is:

• Продавець поклав пучок кропу на ваги.

But you could also say things like:

• На ваги продавець поклав пучок кропу.
• Пучок кропу продавець поклав на ваги.

These versions shift the emphasis:

• На ваги... emphasizes where
• Пучок кропу... emphasizes what

The original sentence is the most straightforward, neutral way to say it.

The cases tell you.

• продавець is nominative, so it is the subject
• пучок is the direct object
• кропу depends on пучок
• на ваги shows direction toward the scales

So even if the words move around, the forms help you understand the structure.

That is one of the big advantages of case systems in Ukrainian.

Then you would usually use на вагах instead of на ваги, because it would describe location, not movement.

For example:

• Пучок кропу лежав на вагах. = The bunch of dill was lying on the scales.
• Пучок кропу був на вагах. = The bunch of dill was on the scales.

So:

• на ваги = onto the scales
• на вагах = on the scales

They are:

• продавець = seller
• покласти = to put, to place
• пучок = bunch
• кріп = dill
• ваги = scales

And the forms in the sentence are:

• продавець — nominative singular
• поклав — past tense, masculine singular, perfective
• пучок — accusative singular
• кропу — genitive singular
• ваги — accusative plural after на showing motion

A useful stress guide is:

• продаве́ць
• покла́в
• пучо́к
• кропу́
• ваги́

A rough pronunciation guide for an English speaker:

• продаве́ць ≈ pro-da-VETS'
• покла́в ≈ po-KLAV
• пучо́к ≈ pu-CHOK
• кропу́ ≈ kro-POO
• ваги́ ≈ va-HY

One important note: Ukrainian г sounds like a softer voiced sound, closer to an English h with voice, not a hard g as in go.