Tapasin erään lääkärin eilen.

Tapasin is the 1st person singular past tense form of the verb tavata, which means to meet.

So:

• tavata = to meet
• tapaan = I meet / I am meeting
• tapasin = I met

The ending -in is a very common marker of the past tense in Finnish for I forms.

Because lääkärin is the form needed for the object in this sentence.

In Tapasin erään lääkärin eilen, the doctor is the thing/person that was met, so it is the object of the verb. In an affirmative sentence like this, when the action is seen as a complete whole, Finnish usually uses the total object.

For singular nouns, the total object often looks like the genitive form:

• lääkäri = doctor
• lääkärin = doctor (object form here)

So lääkärin does not mean possession here. It is functioning as the object form required by the sentence.

Because erään goes with lääkärin, and in Finnish modifiers agree with the noun they modify.

Here:

• eräs = a certain / one
• erään = the matching form used with lääkärin

So:

• erään lääkärin

Both words are in the same case/form because they belong together as one noun phrase.

Erään comes from eräs. In this sentence it means something like:

• a certain
• one
• sometimes simply a/an, depending on context

So erään lääkärin can suggest:

• a certain doctor
• a doctor
• one doctor

The exact English translation depends on context. Compared with plain English a doctor, erään can sound a little more specific or slightly more literary.

Finnish does not have articles, so it normally does not mark indefiniteness the way English does.

That means a bare noun can often be enough in Finnish. But words like eräs or yksi can be used when the speaker wants to make the noun clearly indefinite or introduce it as a particular but not yet identified person.

So English I met a doctor yesterday might be expressed in Finnish in different ways, including:

• Tapasin lääkärin eilen.
• Tapasin erään lääkärin eilen.
• Tapasin yhden lääkärin eilen.

These are similar, but not identical in nuance.

Because this sentence uses a total object, not a partitive object.

Finnish often uses the partitive object when:

• the action is incomplete
• the quantity is indefinite in a certain way
• the sentence is negative

But here the action is a complete event:

• I met a doctor yesterday

That is treated as a whole, completed event, so Finnish uses the total object:

• erään lääkärin

A partitive version such as erästä lääkäriä would not be the normal choice here.

Functionally, it is the object of the sentence, and many learners are told this is the accusative function. But the actual visible form of the noun is the same as the genitive singular.

So the short practical answer is:

• It is the object
• Its form here looks like the genitive singular

This is a very common point of confusion for learners. In modern teaching, people often simply say that singular total objects in affirmative sentences usually take the genitive-looking form.

Because Finnish word order is fairly flexible, and putting eilen at the end is completely natural.

The sentence:

• Tapasin erään lääkärin eilen.

is a neutral way to say it. But Finnish can also move elements around for emphasis or style.

For example:

• Eilen tapasin erään lääkärin. = emphasizes yesterday
• Erään lääkärin tapasin eilen. = stronger emphasis on a certain doctor

So the final position of eilen is normal, but not the only possibility.

Yes, absolutely.

That version is very natural too. The difference is mainly emphasis:

• Tapasin erään lääkärin eilen. = neutral, straightforward statement
• Eilen tapasin erään lääkärin. = highlights yesterday

English also does this sometimes:

• I met a doctor yesterday.
• Yesterday, I met a doctor.

So both are correct; the choice depends on what you want to stress.

Yes, but it can sound a bit more specific or slightly more written/literary than the most neutral everyday option.

In casual speech, a speaker might more often say:

• Tapasin yhden lääkärin eilen.
• Tapasin lääkärin eilen.

The nuance is roughly:

• erään lääkärin = a certain doctor, somewhat more formal or narrative
• yhden lääkärin = one / a doctor, common in speech
• lääkärin = a/the doctor, depending on context

So erään lääkärin is correct and natural, but not always the most conversational choice.

In a negative sentence, Finnish normally uses the partitive object.

So:

• Tapasin erään lääkärin eilen. = I met a certain doctor yesterday.
• En tavannut erästä lääkäriä eilen. = I did not meet a certain doctor yesterday.

Notice the changes:

• tapasin becomes en tavannut
• erään lääkärin becomes erästä lääkäriä

This is a very important Finnish pattern: negative sentence -> partitive object.

Here are the dictionary forms:

• Tapasin -> tavata = to meet
• erään -> eräs = a certain, one
• lääkärin -> lääkäri = doctor
• eilen -> eilen = yesterday

So the sentence is built from very common Finnish patterns:

• verb in the past tense
• object in the total-object form
• time expression eilen

Yes: the word stem changes a little.

The basic form is:

• lääkäri

But in forms like the genitive/object form, it becomes:

• lääkärin

So the final vowel pattern changes slightly. This is normal for many Finnish nouns. Learners often need time to get used to these stem changes.

The important thing to remember here is simply:

• lääkäri -> lääkärin

and to recognize it as the same word, just in a different grammatical form.