Продавщица разменяла купюру и дала мне сдачу мелкими монетами.

Продавщица is a common word for a female shop assistant/saleswoman.
Продавец is the male (or sometimes gender-neutral in some contexts) version meaning salesperson. In everyday speech, Russian often marks the person’s gender like this.

Разменяла = broke / changed (into smaller money).
Купюру = a banknote/bill.
So разменяла купюру means she broke a bill into smaller denominations (often to be able to give change).

Разменяла is perfective past: it presents the action as completed (she did it and it’s done).
Разменивала would be imperfective, suggesting process/repetition/background, e.g. She was changing a bill (when something happened) or She often changed bills. Here the sentence reports a finished sequence of actions, so perfective fits.

Because купюра is the direct object of разменять (to change/break what?):

• nominative: купюра
• accusative singular: купюру
  In Russian, direct objects of transitive verbs commonly take the accusative.

Russian often omits repeated subjects when it’s obvious.
Продавщица разменяла ... и дала ... = The clerk changed ... and (she) gave ...
The subject продавщица applies to both verbs.

Because дать (to give) typically takes:

• direct object (what is given) in the accusative: сдачу
• recipient in the dative (to whom): мне = to me

So: дала (кому?) мне (что?) сдачу.

Сдача means change (money returned after paying).
It’s the direct object of дала, so it’s in the accusative:

• nominative: сдача
• accusative: сдачу

The instrumental here expresses the means/form in which the change was given: “with/by/in small coins.”

• мелкими = instrumental plural of мелкий (small, low-value)
• монетами = instrumental plural of монета (coin)

A very natural alternative is мелочью (instrumental of мелочь), meaning small change:

• ... дала мне сдачу мелочью.

In money contexts, мелкий / мелочь usually means small denomination / small value, not necessarily physically tiny. So мелкими монетами is low-value coins (like pennies/nickels-type amounts).

Yes. Russian word order is flexible, and changes emphasis. For example:

• Продавщица разменяла купюру и дала мне сдачу мелкими монетами. (neutral narrative)
• Мелкими монетами она дала мне сдачу. (emphasizes small coins)
• Мне она дала сдачу мелкими монетами. (emphasizes to me, contrastive)

The core grammar stays the same; the focus shifts.

Common stress patterns:

• продавщи́ца
• разменя́ла
• купю́ру
• сда́чу
• ме́лкими
• моне́тами

Getting these stresses right will make you much easier to understand.