Learn Hardcore Finnish: Joskus tunnen oloni yksinäiseksi, vaikka olen täydessä kahvilassa. - Sometimes I feel lonely even though I am in a full café.

You can’t normally say *tunnen yksinäiseksi on its own; tuntea needs an object or complement like oloni, itseni (“myself”) etc.
Using olen yksinäinen would just be “I am lonely”, without the nuance of “I feel”.

Why is it yksinäiseksi and not yksinäinen or yksinäisen?

Yksinäiseksi is the translative case (ending -ksi). The translative often expresses:

a change into a state, or
how you feel / what something seems like

In expressions with tuntea olonsa/olonsa tms., Finnish almost always uses the translative:

Tunnen oloni väsyneeksi. = I feel tired.
Tunnen oloni huonoksi. = I feel bad.
Tunnen oloni yksinäiseksi. = I feel lonely.

So yksinäinen (nominative) would describe a stable property (“is lonely”),
while yksinäiseksi (translative) fits the pattern “I feel (in) a lonely state”.

What exactly is oloni grammatically? Is it a case form?

Oloni is:

base word: olo = state, condition, how one feels
case: nominative singular (the basic dictionary form)
suffix: -ni = my

So olo + ni → oloni = my state / my condition / how I feel.

It’s not a special case; it’s just a nominative noun with a possessive suffix.
The “feeling” meaning comes from the word olo itself, plus the verb tuntea.

Could you say Joskus olen yksinäinen instead? Is there a difference in meaning?

Yes, you can say Joskus olen yksinäinen, and it’s correct.

Nuance:

Joskus olen yksinäinen.
= Sometimes I am lonely. (more factual, like a description of you)
Joskus tunnen oloni yksinäiseksi.
= Sometimes I feel lonely. (focus on the subjective feeling in that moment)

The original sentence sounds a bit more emotional and introspective, because it emphasizes the internal feeling rather than a stable trait.

What’s the difference between yksinäinen and yksin?

yksinäinen = lonely (emotional state, lack of connection)
yksin = alone (physically by yourself, no one else present)

In this sentence, you need yksinäiseksi (from yksinäinen), because it’s about feeling emotionally lonely even though the café is full of people.

If you said yksin, it would mean you are actually alone, which contradicts “in a full café”.

How does vaikka work here, and why isn’t it something like “even though” + extra word?

In this sentence, vaikka means “even though / although”.

Structure:

… yksinäiseksi, vaikka olen täydessä kahvilassa.
= … lonely, even though I am in a full café.

You don’t need an extra word like että here. Vaikka alone introduces a subordinate clause that expresses contrast:

Lähden ulos, vaikka sataa. = I’m going out, even though it’s raining.
Hän hymyilee, vaikka on surullinen. = She smiles, although she is sad.

So vaikka + finite verb (here olen) is the normal way to say “even though …”.

Why is it olen (present tense) and not something like a past tense form?

Finnish often uses the present tense for:

general, repeated situations (“sometimes when X happens, Y”), and
descriptions that are not tied to one specific past moment.

Joskus tunnen oloni yksinäiseksi, vaikka olen täydessä kahvilassa describes a recurring pattern in your life, not one specific event in the past, so the present tense is used naturally in both clauses.

If you were talking about one particular occasion in the past, you’d shift to past forms, e.g.
Eilen tunsin oloni yksinäiseksi, vaikka olin täydessä kahvilassa.

Why is it täydessä kahvilassa and not *täydessä kahvila?

Both words are inflected for case:

kahvila (café) → kahvilassa = in the café (inessive case, -ssa/-ssä)
täysi (full) → täydessä = in a full (something), inessive singular

The adjective täysi must agree in case and number with the noun it describes:

nominative: täysi kahvila = a full café
inessive: täydessä kahvilassa = in a full café

So *täydessä kahvila is wrong because the cases don’t match.

Does kahvilassa mean “in the café” or “at the café”? How do I know?

Literally, kahvilassa is “in the café” (inside the space), because -ssa/-ssä is the inessive case (“in”).

However, English often says “at the café” where Finnish says “in the café”. So in natural translation, kahvilassa will often be rendered as “at a café” even though the Finnish is literally “in”.

You decide by context which English preposition sounds most natural; the Finnish case itself always means “in(side)” here.

Can I drop oloni and say Joskus tunnen yksinäiseksi?

No, *Joskus tunnen yksinäiseksi is incorrect.

Reasons:

tuntea normally needs an object or complement.
With the “feel a certain way” meaning, Finnish uses structures like:
- tunnen oloni X:ksi
- tunnen itseni X:ksi

So correct options would be:

Joskus tunnen oloni yksinäiseksi.
Joskus tunnen itseni yksinäiseksi. (also common: “I feel myself lonely”)

You need oloni or itseni (or similar) there.

Could I say täydessä kahvilassa olen or move parts of the sentence around? How flexible is the word order?

Finnish word order is fairly flexible, and you can move parts for emphasis. For example:

Joskus, vaikka olen täydessä kahvilassa, tunnen oloni yksinäiseksi.
Vaikka olen täydessä kahvilassa, joskus tunnen oloni yksinäiseksi.

These are all grammatically fine. The basic, neutral word order is:

adverb (Joskus) + verb (tunnen) + object/complement (oloni yksinäiseksi)
then the contrast clause (vaikka olen täydessä kahvilassa)

Putting täydessä kahvilassa at the very start (Täydessä kahvilassa olen…) would sound unusual here and change the focus heavily onto the location.

What’s the difference between joskus and other “sometimes” words like välillä or toisinaan?

All can mean “sometimes”, but there are nuances:

joskus – the most common, neutral “sometimes, at times”.
välillä – literally “between”; as an adverb, often “sometimes / every now and then”, a bit colloquial-feeling in this use.
toisinaan – stylistically a bit more literary or formal; many learners encounter it in written language.

In this sentence, joskus is the most natural, everyday choice:
Joskus tunnen oloni yksinäiseksi… = Sometimes I feel lonely…

Could I say ruuhkaisessa kahvilassa instead of täydessä kahvilassa? Would it change the meaning?

Yes, you could say ruuhkaisessa kahvilassa, and it would be correct.

täydessä kahvilassa = in a full café (every seat is taken / very crowded)
ruuhkaisessa kahvilassa = in a busy / congested café (lots of movement/traffic, crowded)

Both express that there are many people around. The emotional point of the sentence stays the same; only the nuance of what kind of crowd it is changes slightly.