Maksuaika loppuu perjantaina.

Because Finnish very often forms compound nouns by joining words together.

• maksu = payment
• aika = time, period

So maksuaika means something like payment time / payment period / time allowed for payment.

This is much more natural in Finnish as one word than as two separate words.

In this sentence, maksuaika means the time limit for paying, payment period, or due period.

It does not mean just time in a general sense. It refers to the allowed period during which a payment should be made.

So in practical contexts, it often functions like the payment deadline period or the due time.

Because loppua and lopettaa are different verbs.

• loppua = to end, to come to an end, to run out
• lopettaa = to end/stop something, to quit

In this sentence, maksuaika is the thing that ends by itself, so Finnish uses loppua:

• Maksuaika loppuu = The payment period ends

If you used lopettaa, it would suggest that someone ends something:

• Yritys lopettaa palvelun. = The company ends the service.

So loppuu is correct because the subject is not actively ending something else.

The basic verb is loppua. In the 3rd person singular present tense, it becomes loppuu.

That form is used because the subject maksuaika is singular:

• maksuaika loppuu = the payment period ends

A few examples:

• Elokuva loppuu. = The movie ends.
• Kesä loppuu. = Summer ends.

So -uu is simply part of how this verb is conjugated in that form.

Because Finnish uses a special case ending here for days of the week used as time expressions.

• perjantai = Friday
• perjantaina = on Friday

The ending -na / -nä is the essive case, and with days it commonly expresses when something happens.

So:

• Maksuaika loppuu perjantaina. = The payment period ends on Friday.

Other examples:

• maanantaina = on Monday
• tiistaina = on Tuesday
• sunnuntaina = on Sunday

Grammatically, perjantaina most directly means on Friday.

So the sentence says that the payment period ends on Friday.

In real-life usage, this often implies that Friday is the deadline, which may feel similar to by Friday in English. But the Finnish form itself is still on Friday, not literally by Friday.

If you wanted to say by Friday more explicitly, Finnish could use something like:

• perjantaihin mennessä = by Friday

So:

• Maksuaika loppuu perjantaina = the payment period ends on Friday
• Maksa perjantaihin mennessä = pay by Friday

Because Finnish does not have articles like a, an, and the.

So maksuaika can mean:

• a payment period
• the payment period

The exact meaning depends on the context.

In this sentence, English naturally uses the:

• The payment period ends on Friday

But Finnish does not need a separate word for that.

Maksuaika loppuu perjantaina is the most neutral and straightforward word order:

• maksuaika = subject
• loppuu = verb
• perjantaina = time expression

Finnish word order is quite flexible, but changing it changes the emphasis.

For example:

• Maksuaika loppuu perjantaina. = neutral statement
• Perjantaina maksuaika loppuu. = emphasizes Friday
• Loppuu maksuaika perjantaina. = unusual in normal context

So the given order is simply the most natural default.

Yes. That is grammatical.

It means the same basic thing, but it puts more focus on perjantaina:

• Perjantaina maksuaika loppuu. = On Friday, the payment period ends.

This kind of word order is common when you want to highlight the time.

So both are correct, but:

• Maksuaika loppuu perjantaina. = neutral
• Perjantaina maksuaika loppuu. = more emphasis on Friday

No. There is no object here.

That is because loppua is an intransitive verb in this meaning. It does not take an object.

The sentence structure is simply:

• maksuaika = subject
• loppuu = verb
• perjantaina = adverbial expression of time

So the sentence is complete without any object.

If the subject were plural, both the noun and the verb would usually change.

For example:

• Maksuajat loppuvat perjantaina. = The payment periods end on Friday.

Here:

• maksuajat = plural of maksuaika
• loppuvat = 3rd person plural of loppua

So in the original sentence, loppuu is singular because maksuaika is singular.

Yes, the double letters matter a lot in Finnish.

In loppuu:

• pp means the consonant is held longer
• uu means the vowel is held longer

So Finnish distinguishes between short and long sounds, and that can change meaning.

You should pronounce loppuu with a clearly long p sound and a clearly long u sound. Not exactly like English spelling habits.

This is an important general rule in Finnish:

• single letter = short sound
• double letter = long sound

Sometimes in practice, yes, but not exactly in the strictest sense.

• maksuaika = the period allowed for payment
• deadline is often more like a specific final point in time

A more exact Finnish word for deadline is often:

• määräaika = deadline / fixed deadline

Still, in many everyday payment contexts, maksuaika loppuu perjantaina effectively tells you when the deadline is, so the practical idea is very close.