Stavi šalicu na tanjurić da ne ostane mrlja na stolu.

Because šalicu is the accusative singular of šalica.

In this sentence, šalicu is the direct object of Stavi: it is the thing you are putting somewhere.

• šalica = nominative, the basic dictionary form
• šalicu = accusative singular

A very common pattern in Croatian is:

• Stavi knjigu na stol. = Put the book on the table.
• Uzmi olovku. = Take the pencil.

So here:

• Stavi šalicu... = Put the cup...

Because the preposition na changes case depending on whether it shows movement or location.

1. na + accusative = movement onto something

In na tanjurić, the cup is being moved onto the saucer.

• tanjurić is masculine inanimate
• its accusative singular is the same as its nominative singular
• so na tanjurić means onto the saucer

2. na + locative = location on something

In na stolu, the stain would be on the table, already located there.

• stol = table
• stolu = locative singular
• na stolu = on the table

So the contrast is:

• Stavi šalicu na tanjurić = move it onto the saucer
• mrlja na stolu / na stolu = a stain located on the table

Here da introduces a purpose clause: so that, so, in order that.

So:

• da = so that
• ne = not
• ostane = remains / is left

Very literally, the second half means:

• so that a stain does not remain on the table

In more natural English, that becomes something like:

• so that it doesn’t leave a mark on the table
• so there won’t be a stain left on the table

So da ne ostane expresses the goal of the action: put the cup on the saucer to prevent a stain.

Because mrlja is the subject of ostane, not the object.

This is an important difference from English.

In English, we often say:

• leave a stain

where stain looks like the object.

But Croatian uses the verb ostati = to remain / to be left, so the structure is more like:

• so that a stain does not remain on the table

That makes mrlja the subject, so it stays in the nominative:

• mrlja = nominative singular

If it were a direct object, then you would expect a different case, but that is not how ostati works.

Stavi is the imperative singular of the verb staviti.

It means:

• Put!
• Place!

It is used when speaking to one person informally.

Related forms:

• stavi = singular informal command
• stavite = plural or formal command

So:

• Stavi šalicu... = Put the cup...
• Stavite šalicu... = Put the cup... / Please put the cup...

This is about aspect, which is very important in Croatian.

• staviti = perfective
• stavljati = imperfective

Stavi

This sounds like a single completed action:

• put it down
• place it once

That is exactly what fits here.

Stavljaj

This would suggest something more like:

• keep putting
• put repeatedly
• be in the habit of putting

So in this sentence, Stavi is the natural choice because the speaker wants one concrete action with a result.

Again, this is aspect.

• ostati → ostane = perfective
• ostajati → ostaje = imperfective

Here the speaker is talking about a single result to avoid:

• a stain being left behind

So Croatian naturally uses the perfective form ostane in the da-clause:

• da ne ostane mrlja = so that no stain is left

If you used ostaje, it would sound more ongoing or habitual, which does not fit as well here.

So both main verbs point toward one complete event:

• Stavi the cup
• so that a stain does not ostane

Yes, historically it is a diminutive of tanjur.

• tanjur = plate
• tanjurić = little plate

But in everyday Croatian, tanjurić is also the normal word for a saucer, especially the small plate under a cup.

So in this sentence, na tanjurić is best understood as:

• onto the saucer

Even though the form looks diminutive, it is very natural and standard here.

Because Croatian does not have articles like English.

So Croatian nouns can mean:

• a cup
• the cup

depending on context.

That means:

• šalicu can mean a cup or the cup
• tanjurić can mean a saucer or the saucer
• mrlja can mean a stain or the stain

If Croatian needs to be more specific, it can use words like:

• taj = that
• ovaj = this

For example:

• Stavi tu šalicu na taj tanjurić. = Put that cup on that saucer.

Croatian word order is fairly flexible, but the original sentence is natural and neutral.

• Stavi šalicu na tanjurić da ne ostane mrlja na stolu.

You can move things around for emphasis, for example:

• Stavi šalicu na tanjurić da na stolu ne ostane mrlja.

This puts slightly more focus on na stolu.

You could also say:

• Na tanjurić stavi šalicu...

if you want to emphasize the saucer.

But for a straightforward instruction, the original order is probably the most normal.