Learn Hardcore Russian: Иногда я пишу в дневнике, за что я сегодня благодарен. - Sometimes I write in my diary what I am grateful for today.

Whenever you have a main clause plus a clause introduced by что, который, где, когда, потому что, если, etc., you almost always put a comma before that second clause in Russian.

What exactly does за что mean here, and why is it за, not о чём or чему?

За что here literally means “for what” in the sense of “for what reason / for what thing.”

The adjective благодарен (“grateful”) normally takes:

кому / чему – to whom you are grateful
за что – for what you are grateful

Examples:

Я благодарен тебе за помощь. – I am grateful to you for your help.
- тебе (кому?) – to whom
- за помощь (за что?) – for what

In your sentence, the thing you’re grateful for is not named directly; instead, it’s a whole clause:

за что я сегодня благодарен – what I am grateful for today

Using о чём would change the meaning:

писать о чём = to write about something (topic)
быть благодарным за что = to be grateful for something (reason)

So за is required by the word благодарен, not by писать. The diary entry is about that for which I am grateful today, hence за что.

Why is it я … благодарен and not something like я благодарный?

Russian has two forms of many adjectives:

long form: благодарный
short form: благодарен

They are related but used differently:

Short form (благодарен)
- Describes a current state / feeling.
- Often used like “to be X”:
  - Я благодарен. – I am grateful.
- Common in statements of emotion or evaluation.
Long form (благодарный)
- More like a general characteristic (“a grateful person”), or used attributively:
  - благодарный человек – a grateful person
  - Он всегда был очень благодарным учеником. – He was always a very grateful student.

In your sentence, we are talking about how the person feels today, not about a permanent quality of character, so the short form благодарен is natural:

… за что я сегодня благодарен. – … for what I am (feeling) grateful today.

Why is благодарен masculine if the subject is я, which doesn’t show gender? What would a woman say?

In Russian, short-form adjectives (like благодарен) agree with the gender of the person or thing they describe.

With я, the form shows the speaker’s gender:

A man:
- Я сегодня благодарен.
A woman:
- Я сегодня благодарна.
A neuter subject (less common in real speech, more in grammar examples):
- Это чувство мне благодарно.

So your sentence as written:

Иногда я пишу в дневнике, за что я сегодня благодарен.

implicitly assumes the speaker is male.
A female speaker would say:

Иногда я пишу в дневнике, за что я сегодня благодарна.

Why is there no word for “am” (like есть) before благодарен? How is the tense expressed?

In Russian, in the present tense, the verb “to be” (быть) is usually omitted in sentences like this:

Я студент. – I am a student.
Он дома. – He is at home.
Я сегодня благодарен. – I am grateful today.

So the structure я благодарен already means “I am grateful”; adding есть would sound archaic or stylistically marked in modern normal speech.

The tense is understood from:

the form of благодарен (short-form adjective in present state), and
the time adverb сегодня (“today”)

So я сегодня благодарен = “I am (feeling) grateful today,” not “I was” or “I will be.”

Why is it в дневнике and not в дневник? What case is дневнике?

Дневнике is prepositional case singular of дневник.

Nominative: дневник
Prepositional: в дневнике – in the diary

The preposition в can take either:

prepositional = “in, inside (location)”
accusative = “into, to (direction)”

With verbs of writing, both are possible, but they have slightly different nuances:

писать в дневнике (prepositional – location)
- Literally “to write in the diary”
- Focus on the place where the writing exists.
писать в дневник (accusative – direction/result)
- Literally “to write into the diary”
- Focus on the diary as the target of the writing.

In everyday speech, both are common and often interchangeable.
Your sentence uses the “location” idea:

Иногда я пишу в дневнике… – Sometimes I write in my diary…

Why is the verb пишу (imperfective) used here and not напишу or запишу?

Пишу is the imperfective present form of писать and is used for:

habitual / repeated actions
ongoing processes

That fits иногда (“sometimes”): a repeated habit.

Aspect contrast:

писать (пишу) – imperfective
- I write / I am writing (as a general habit or ongoing)
- Иногда я пишу… – Sometimes I write…
написать (напишу) – perfective
- I will write (one complete act)
- Когда‑нибудь я напишу дневник. – One day I will write a diary (once, fully).
записать (запишу) – perfective, “to write down, to note down”
- Focus on recording something specific, once:
- Я запишу, за что я сегодня благодарен. – I’ll write down what I am grateful for today (this one time).

Your sentence describes a general habit: sometimes I do this, sometimes I don’t.
So the imperfective пишу is the natural choice.

Can I move иногда or сегодня around? For example: Иногда я сегодня пишу… or Я иногда пишу…?

You have some flexibility, but not all positions sound natural.

Most natural options:

Иногда я пишу в дневнике, за что я сегодня благодарен.
Я иногда пишу в дневнике, за что я сегодня благодарен.
Иногда я пишу в дневнике, за что сегодня я благодарен.
Иногда я пишу в дневнике, за что я благодарен сегодня.

All of these are acceptable, with only slight changes in emphasis.

Less natural or odd:

Иногда я сегодня пишу… – sounds strange, because иногда (“sometimes”) and сегодня (“today”) conflict: “sometimes today” is not a typical combination.

General guidelines:

Иногда: usually near the beginning or after the subject:
- Иногда я… / Я иногда…
Сегодня: usually close to the verb/adjective it modifies:
- я сегодня благодарен / я благодарен сегодня

So prefer positions like in the original sentence or variants (2–4) above.

Could I omit the second я and say: …за что сегодня благодарен?

Yes, you can omit it; Russian often drops repeated subjects when they are clear from context:

Иногда я пишу в дневнике, за что сегодня благодарен.

This is grammatically correct and understandable: “Sometimes I write in my diary what (for what) I am grateful for today.”

Style nuance:

With я included:
- …за что я сегодня благодарен.
  Slightly more explicit and neutral.
Without я:
- …за что сегодня благодарен.
  Sounds a bit more compact, perhaps a bit more literary.

Both are fine; many speakers would naturally keep я in spoken language.

What exactly does дневник mean here? Is it the same as English “journal” or “magazine”?

In this context, дневник means a personal diary, something you write in regularly about your thoughts, feelings, events of the day.

Common meanings of дневник:

Personal diary / journal – as in your sentence.
School record book – a booklet where teachers write grades and homework (in some post‑Soviet school systems).

By contrast:

журнал usually means:
- a magazine, or
- a logbook / register (e.g. журнал посещаемости – attendance log)

So here дневник = a private diary / journal you write for yourself, not a magazine.