Jätän suuren matkatavaran tiskille, mutta pidän repun mukana.

Because suuren is the form that agrees with matkatavaran in this sentence.

The basic form is:

• suuri = big, large

But here matkatavaran is in the object form often called the genitive/accusative-looking total object, so the adjective changes too:

• suuri matkatavara = a large piece of luggage / large luggage
• suuren matkatavaran = the large piece of luggage (as the complete object of the verb)

In Finnish, adjectives usually match the noun in case and number, so if the noun changes form, the adjective changes too.

Here matkatavaran is the object of jätän (I leave), and it appears in the total object form.

Compare:

• Jätän matkatavaran tiskille. = I leave the luggage at the counter.
• Jätän matkatavaraa tiskille. = I am leaving some luggage / luggage in an incomplete or unspecified amount.

In this sentence, the speaker is leaving a specific, complete item or amount, so Finnish uses the total-object form matkatavaran.

Also, matkatavara is often a collective word, a bit like English luggage or baggage, so even though it looks singular, it can refer to luggage in general.

Matkatavara means luggage, baggage, travel belongings.

It is often used as a collective noun, much like English luggage, which is grammatically singular but refers to multiple things.

So:

• matkatavara = luggage/baggage
• matkatavarat = pieces of luggage / belongings, more explicitly plural

In this sentence, the singular matkatavaran sounds natural if the speaker means their large luggage as one unit. If you wanted to be more concrete, you might also hear words like:

• matkalaukku = suitcase
• laukku = bag

Because -lle expresses movement to something.

• tiski = counter, desk
• tiskille = to the counter
• tiskillä = at the counter
• tiskiltä = from the counter

Since the sentence says the speaker is leaving the luggage to/at the counter, Finnish uses the form that marks movement toward the place:

• Jätän ... tiskille = I leave ... at/to the counter

English often just says at the counter, but Finnish makes a clearer distinction between:

• motion toward something → -lle
• location at something → -llä

For the same basic reason as matkatavaran: it is the object of the verb pidän in the sense of I keep.

• reppu = backpack
• repun = backpack as a total object

So:

• pidän repun mukana = I keep the backpack with me

This suggests the backpack is a specific whole item that the speaker is keeping with them.

Mukana means with me / along / carried with me depending on context.

In this sentence:

• pidän repun mukana = I keep the backpack with me

It is a very common Finnish word in travel and everyday situations. For example:

• Onko passi mukana? = Do you have your passport with you?
• Otan veden mukaan. = I’ll take water with me.
• Pidän laukun mukana. = I’ll keep the bag with me.

So mukana is not just a plain word for with in the same way as kanssa. It specifically gives the idea of something being carried along or kept on one’s person.

Because in Finnish, mukana can imply the person from the verb, especially when the meaning is obvious.

• pidän repun mukana = I keep the backpack with me

The I is already clear from pidän, so Finnish does not need to say with me separately.

If you wanted to be more explicit, Finnish often uses a possessive suffix with words like this:

• pidän repun mukanani = I keep the backpack with me

That version is slightly more explicit or polished. But mukana alone is very common in everyday speech when the meaning is clear from context.

They describe two opposite actions:

• jätän = I leave behind / I leave somewhere
• pidän = I keep / I hold onto

So the sentence contrasts two choices:

• Jätän suuren matkatavaran tiskille = I leave the large luggage at the counter
• mutta pidän repun mukana = but I keep the backpack with me

This is a very natural pairing in travel situations: one thing is checked, stored, or left somewhere, while another is kept with the traveler.

Yes, pitää has more than one meaning.

Two very common meanings are:

1. pitää jostakin = to like something

   • Pidän kahvista. = I like coffee.

2. pitää jokin = to keep/hold something

   • Pidän laukun mukana. = I keep the bag with me.

In your sentence, it is clearly the second meaning: to keep.

A useful clue is the grammar:

• pitää jostakin uses -sta/-stä
  
  • pidän repusta = I like the backpack
• pitää jokin mukana uses a direct object
  
  • pidän repun mukana = I keep the backpack with me

Yes, Finnish word order is fairly flexible, but this version is the most neutral and natural.

The structure is:

• Jätän suuren matkatavaran tiskille
• mutta pidän repun mukana

This gives a clear contrast:

• first action: leave the large luggage
• second action: keep the backpack

You could move things around for emphasis, for example:

• Repun pidän mukana, mutta jätän suuren matkatavaran tiskille.

That would emphasize the backpack. But the original sentence is the most straightforward, neutral way to say it.

Mutta means but.

It links the two contrasting parts of the sentence:

• I leave the large luggage at the counter,
• but I keep the backpack with me.

It is one of the most common Finnish conjunctions.

Yes. The choice depends on how specific you want to be.

• matkatavara = luggage/baggage in a general or collective sense
• laukku = bag
• matkalaukku = suitcase

So if the speaker specifically means one large suitcase, many learners may expect something like:

• Jätän suuren matkalaukun tiskille, mutta pidän repun mukana.

That would mean:

• I leave the large suitcase at the counter, but keep the backpack with me.

The original sentence with matkatavaran is still possible; it just sounds a bit more general, like large luggage/baggage.

Yes. For learners, this is one of the most common patterns to notice.

Both nouns are objects in the sentence, and both appear with -n:

• matkatavara → matkatavaran
• reppu → repun

In many basic Finnish sentences, when the object is understood as a complete whole, it takes this -n form.

That is why English speakers often first learn it as a kind of object form, even though grammatically it overlaps with the genitive form.

So this sentence is a good example of a very common Finnish pattern:

• complete object → often -n

Usually mukana is better here.

• mukana = with me, along with me, in my possession
• kanssa = with, together with

So:

• Pidän repun mukana. = I keep the backpack with me.
• Pidän repun kanssani. = grammatically possible in some contexts, but not the natural choice here

For carrying or keeping an item on your person, mukana is the normal word.

A good rule:

• for having something with you → mukana
• for being together with someone/something → often kanssa

A few close everyday alternatives are possible, depending on what exactly is meant.

For example:

• Jätän ison matkalaukun tiskille, mutta pidän repun mukana.
• Jätän ison laukun tiskille, mutta otan repun mukaan.

These sound a bit more concrete or colloquial because:

• iso is often more everyday than suuri
• matkalaukku is more specific than matkatavara
• otan ... mukaan = I take ... with me, which is also very common

Your original sentence is perfectly good Finnish; these are just useful comparison points for what learners may hear in real life.