Learn Hardcore Finnish: Pidän pientä päiväkirjaa, jotta muisti pysyy hyvänä. - I keep a small diary so that my memory stays good.

So Pidän pientä päiväkirjaa already means “I keep a small diary”.
You can say Minä pidän for emphasis (like “I keep…”), but it’s not needed.

What exactly does the verb pitää mean here? I thought it meant “to like”.

Pitää is a very polysemous verb. Common meanings:

pitää + -sta/-stä = to like
- Pidän kahvista. = I like coffee.
pitää (jotakin) = to hold / to keep
- Pidän pientä päiväkirjaa. = I keep a small diary.
- Pidän ovea auki. = I keep the door open.
pitää (tehdä jotakin) = must / have to (spoken language)
- Minun pitää lähteä. = I must go.

In this sentence it’s sense 2: “to keep / maintain” a diary as an ongoing activity.

Why is it pientä päiväkirjaa and not pieni päiväkirja?

Because pientä päiväkirjaa is in the partitive singular, not the nominative.

pieni päiväkirja = basic dictionary form (nominative singular)
pientä päiväkirjaa = partitive singular (adjective and noun agree in case)

Reasons for the partitive here:

With pitää in the sense “to keep (something) as a habit”, the object is typically in the partitive because the action is ongoing / not completed.
- Pidän pientä päiväkirjaa. = I keep a small diary (as a regular activity).
The diary is indefinite (“a” diary, not some specific whole diary that has a clear end), and this kind of habitual, open‑ended activity often uses the partitive object.

If you used pienen päiväkirjan (genitive object), it would suggest a total, completed object — which really doesn’t fit well with “keeping a diary” as a continuous habit.

So is pientä just agreeing with päiväkirjaa in case and number?

Yes.

Finnish adjectives agree with the noun they describe in:

case
number
(and possessive endings if there are any)

Here:

päiväkirjaa = partitive singular of päiväkirja
pientä = partitive singular of pieni

So you get: pientä päiväkirjaa = a small diary (as a partitive object).

Could you say Pidän pienen päiväkirjan? What would that mean?

Grammatically it’s possible, but the meaning changes and it sounds odd in normal usage.

pienen päiväkirjan = genitive singular object (a total object)

With a verb like pitää (“to keep/hold”), a total object would suggest that the action has a clear endpoint, similar to finishing or handling the whole thing.

So Pidin pienen päiväkirjan (past tense) could be interpreted as something like:

I kept a small diary (for a specific limited time and then it was over)

Even then, it’s unnatural; native speakers would normally express that with some other wording. For the habitual sense “I keep a small diary”, you need partitive: Pidän pientä päiväkirjaa.

Why is there a comma before jotta?

Jotta starts a subordinate clause that expresses purpose (“so that / in order that”). In Finnish, it’s standard to put a comma before such a clause:

Pidän pientä päiväkirjaa, jotta muisti pysyy hyvänä.
I keep a small diary, so that my memory stays good.

More examples:

Teen muistiinpanoja, jotta muistan kaiken.
Opiskelen ahkerasti, jotta pääsen kokeesta läpi.

The comma marks the boundary between the main clause and the purpose clause.

What does jotta mean exactly, and how is it different from että or koska?

Jotta means “so that / in order that” and introduces a purpose or desired result.

Pidän pientä päiväkirjaa, jotta muisti pysyy hyvänä.
I keep a small diary so that my memory stays good.

Differences:

jotta = purpose / aim
- Teen töitä, jotta saan rahaa.
  I work so that I get money.
että = “that” (content clause, reported speech, etc.)
- Tiedän, että se on vaikeaa.
  I know that it is difficult.
koska = “because” (reason / cause)
- Pidän päiväkirjaa, koska haluan muistaa asiat.
  I keep a diary because I want to remember things.

In this sentence, we’re talking about the goal (keeping memory good), not the cause, so jotta is the natural choice.

Why is it muisti pysyy hyvänä and not muisti pysyy hyvä?

Because with pysyä (“to stay / remain”), the complement is typically in the essive case (-na / -nä), not plain nominative:

pysyä + essive = to remain in/as some state

Examples:

Muisti pysyy hyvänä. = Memory stays good.
Vesi pysyy kylmänä. = The water stays cold.
Hän pysyi rauhallisena. = He/She remained calm.

So hyvä (nominative) → hyvänä (essive).
Muisti pysyy hyvä would be incorrect in standard Finnish.

What case is hyvänä, and what is the basic form of this word?

Basic form (nominative): hyvä = good
Form in the sentence: hyvänä = essive case

The essive (-na/-nä) often expresses:

being in a certain role or state:
- Opiskelen opettajana. = I work as a teacher.
a temporary or current state:
- Hän on tänään väsynytnä. (less common, but possible)
with pysyä / säilyä etc., to express remaining in some state:
- muisti pysyy hyvänä
- ruoka säilyy tuoreena = the food stays fresh

Why is muisti without any possessive like “my memory”? How do we know it means my memory?

Finnish often uses a bare noun where English uses a possessive, especially for:

body parts
personal qualities
things clearly belonging to the subject

Context tells us whose memory we are talking about. Since the speaker is keeping a diary about their own life, it’s naturally understood as their own memory.

If you want to be explicit, you can say:

Pidän pientä päiväkirjaa, jotta muistini pysyy hyvänä.
(my memory)

But muisti alone is very natural here and is usually understood as “my memory”.

Could the word order be Muisti pysyy hyvänä, joten pidän pientä päiväkirjaa? Would that change the meaning?

Yes, you can change the order and it’s still correct Finnish:

Muisti pysyy hyvänä, joten pidän pientä päiväkirjaa.

Changes:

Focus / emphasis:
- Original: emphasizes the action (keeping a diary) and then the purpose.
- New: starts with the result/state of memory and then gives the reason.
Conjunction:
- jotta = “so that” (purpose)
- joten = “so / therefore” (logical consequence)

So the meaning shifts slightly:

Original: I keep a diary *in order that my memory stays good.*
Alternative: My memory stays good, *so I keep a diary.* (which is actually a bit strange logically — you’d expect the diary to be the cause of good memory, not the other way round).

So from a purely grammatical standpoint the word order is flexible, but you also changed jotta → joten, which changes the logical relationship.

Is pidän pientä päiväkirjaa more like “I keep a diary” or “I am keeping a diary (right now)”?

Pidän pientä päiväkirjaa most naturally expresses a habitual, ongoing activity, so it matches English:

“I keep a small diary.”
(something you do regularly, part of your routine)

Finnish doesn’t distinguish simple present vs present continuous the way English does. Context decides whether it feels like:

“I keep a diary” (habit), or
“I’m keeping a diary (these days)” (current project)

Here, together with the purpose clause jotta muisti pysyy hyvänä, it clearly sounds like a regular habit to support memory.