Eräs asiakas soitti aamulla.

Eräs means a certain, one, or sometimes just some in the sense of one particular but unspecified person or thing.

In this sentence, Eräs asiakas means something like:

• a certain customer
• one customer
• some customer

It often introduces someone who is not identified by name, but who is still a specific person in the speaker’s mind.

So eräs is a bit more specific than just a plain English a/an.

Although both can relate to one, they are not always used the same way.

• yksi usually emphasizes the number one
• eräs usually means a certain / one particular

Compare the nuance:

• Yksi asiakas soitti aamulla. = One customer called in the morning
  This can emphasize the number: one customer, not two or three.

• Eräs asiakas soitti aamulla. = A certain customer called in the morning
  This sounds more like the speaker is introducing an unspecified individual.

So in this sentence, eräs is not mainly about counting. It is about referring to one specific but unnamed customer.

Asiakas is the subject of the sentence, so it appears in the nominative singular, which is the basic dictionary form.

Here:

• eräs asiakas = the subject
• soitti = the verb
• aamulla = a time expression

Because the customer is the one doing the action, Finnish uses the nominative:

• asiakas

There is no ending added here because singular subjects in ordinary affirmative sentences are usually in the nominative.

Soitti is the past tense, third person singular form of the verb soittaa.

Breakdown:

• dictionary form: soittaa
• stem used here: soitti-
• person ending for he/she/it in the past: zero ending in this form
• result: soitti = called

So Eräs asiakas soitti means A customer called or One customer called.

Soittaa is a very common Finnish verb with several related meanings. Depending on context, it can mean:

• to call / phone
• to play an instrument
• to ring or cause something to sound

In this sentence, because the subject is a customer, the natural meaning is to call / phone.

Examples of other uses:

• Hän soittaa pianoa. = He/She plays the piano.
• Soitan sinulle huomenna. = I’ll call you tomorrow.

So the verb has more than one meaning, and context tells you which one is intended.

Aamulla is the word aamu in the adessive case.

• aamu = morning
• aamulla = in the morning / this morning / during the morning

The ending -lla/-llä is often taught first as meaning on or at, but Finnish also uses it in many time expressions.

So here:

• aamulla = in the morning

This is very common with parts of the day:

• aamulla = in the morning
• päivällä = in the daytime / during the day
• illalla = in the evening
• yöllä = at night

So this is not literally about physical location. It is a normal Finnish way to express time.

That is a very reasonable question, because Finnish uses several cases for time expressions.

Aamulla is the normal everyday way to say in the morning.

Aamuna is possible in some contexts, but it is more specific and often means on the morning of some particular day or occasion.

For example:

• Hän soitti aamulla. = He/She called in the morning.
  This is the ordinary, natural form.

• Jouluaamuna satoi lunta. = On Christmas morning, it snowed.
  Here aamuna is tied to a specific named morning.

So in your sentence, aamulla is the expected choice.

In a way, yes—but only partly.

Finnish has no articles like English a/an or the. That means a noun can often appear without any word corresponding to an article.

For example:

• Asiakas soitti. could mean A customer called or The customer called, depending on context.

By adding eräs, the speaker gives extra nuance:

• Eräs asiakas soitti. = A certain customer called / One customer called

So eräs is not a true article, but in translation it can sometimes feel similar to English a.

Yes. Finnish word order is fairly flexible, although not completely free.

The neutral order here is:

• Eräs asiakas soitti aamulla.

But you could also say:

• Aamulla eräs asiakas soitti.

This puts more focus on aamulla: In the morning, a customer called.

You might also hear:

• Eräs asiakas aamulla soitti.

But that sounds less neutral and more marked or stylistically unusual in ordinary speech.

So the original sentence is the most straightforward basic order: subject + verb + time expression.

Not by itself.

Eräs asiakas means a certain customer / one customer, not specifically one of the customers.

If you wanted to say one of the customers, Finnish would usually say something like:

• Yksi asiakkaista

That literally means one from the customers.

So:

• Eräs asiakas = a certain customer
• Yksi asiakkaista = one of the customers

Those are different ideas.

Because here soittaa is being used in the sense to call / phone, and the sentence is complete without stating whom the customer called.

English does the same:

• A customer called in the morning.

You do not have to say called us or called the office, unless that information matters.

If you wanted to include the person or place called, Finnish could add it:

• Eräs asiakas soitti meille aamulla. = A customer called us in the morning.
• Eräs asiakas soitti toimistoon aamulla. = A customer called the office in the morning.

So the original sentence is perfectly complete as it is.

It is neutral standard Finnish.

Nothing in it is especially formal or especially casual. It would fit well in:

• narration
• written reports
• normal conversation
• customer-service contexts

In very casual speech, some forms might change in pronunciation or wording, but the sentence itself is standard and natural.

Then both the subject and the verb would change.

Singular:

• Eräs asiakas soitti aamulla. = A certain customer called in the morning.

Plural:

• Eräät asiakkaat soittivat aamulla. = Certain customers called in the morning.

Changes:

• eräs → eräät
• asiakas → asiakkaat
• soitti → soittivat

So Finnish marks number not only on the noun, but also on the verb.