Learn Hardcore Russian: Я благодарен другу за помощь.

Я благодарен ≈ “I am grateful / I am thankful.”
It describes a state of the subject (я), just like English “I am happy / I am tired / I am grateful.”

The verb “to thank” is благодарить (e.g. я благодарю – “I thank”).
So:

Я благодарен другу за помощь. = “I am grateful to my friend for the help.”
Я благодарю друга за помощь. = “I thank my friend for the help.”

Why does благодарен end with -ен and not with -ый like a normal adjective?

Because благодарен is a short-form adjective.
Russian has two main types of adjective forms:

Full forms (long forms):
- Masculine: благодарный
- Feminine: благодарная
- Neuter: благодарное
- Plural: благодарные
Short forms (what we see in the sentence):
- Masculine: благодарен
- Feminine: благодарна
- Neuter: благодарно
- Plural: благодарны

General tendencies:

Short forms are used mainly in the predicate, to describe a current state:
- Я благодарен. – I am (currently) grateful.
- Он доволен. – He is satisfied.
Full forms are used mostly before nouns or to talk about a more permanent quality:
- Он благодарный человек. – He is a grateful person (as a character trait).

So благодарен (not благодарный) is natural in a sentence like Я благодарен другу…

How does the form of благодарен change if the speaker is female, or if it’s “we” or “they”?

The short-form adjective agrees with the gender and number of the subject:

Masculine singular: Я благодарен другу за помощь.
“I (male) am grateful to my friend for the help.”
Feminine singular: Я благодарна другу за помощь.
“I (female) am grateful to my friend for the help.”
Neuter singular (for neuter nouns like оно): Оно благодарно другу за помощь.
Plural (we, you-plural, they): Мы благодарны другу за помощь.
“We are grateful to our friend for the help.”
Они благодарны другу за помощь. – “They are grateful…”

So when you speak about yourself, you choose благодарен or благодарна depending on your own gender in Russian.

Why is it другу and not друг?

Другу is in the dative case (singular).
The dative case often answers “to whom? / for whom?”

The adjective благодарен (like рад, признателен) is normally followed by a noun/pronoun in the dative to show to whom you are grateful:

Я благодарен другу. – I am grateful to (my) friend.
Я благодарен тебе. – I am grateful to you (informal, singular).
Я благодарен маме. – I am grateful to (my) mom.

So:

друг – nominative (dictionary form)
другу – dative singular (“to the friend”)

In this sentence, the structure is:

Я (кто?) – nominative (subject)
благодарен (кому?) другу – dative (person to whom you are grateful)

What case is помощь in, and why do we say за помощь?

Here помощь is in the accusative case, after the preposition за.

Nominative singular: помощь (help)
Accusative singular: помощь (same form for this feminine noun)

The preposition за can take the accusative to mean “for (something)” in the sense of reason for gratitude, praise, punishment, etc.:

Спасибо за помощь. – Thank you for the help.
Его похвалили за работу. – He was praised for the work.
Его наказали за ошибку. – He was punished for the mistake.

So за помощь literally is “for (the) help,” and the noun is in the accusative, because that’s how за works in this meaning.

Why за помощь and not для помощи or just помощь?

Different prepositions in Russian express different relationships:

за + accusative here means “for (as a reason / cause)”:
- Я благодарен другу за помощь. – I am grateful to my friend for the help (because he helped me).
для + genitive means “for (intended for, for the purpose of)”:
- лекарство для детей – medicine for children (intended for them)
- деньги для ремонта – money for the repair

If you said Я благодарен другу для помощи, it would sound wrong: для does not express the cause of gratitude.

Using just помощь without за (e.g. Я благодарен другу помощь) is ungrammatical. In this construction, you must use за + accusative to express “for what” you are grateful.

Can I replace другу with a pronoun like “you,” “him,” “her,” etc.? How do the forms change?

Yes, but remember you need the dative forms of the pronouns:

Я благодарен тебе за помощь. – I am grateful to you (informal, singular) for the help.
Я благодарен вам за помощь. – I am grateful to you (plural or polite singular) for the help.
Я благодарен ему за помощь. – I am grateful to him for the help.
Я благодарен ей за помощь. – I am grateful to her for the help.
Я благодарен им за помощь. – I am grateful to them for the help.

These are all dative forms (кому?): тебе, вам, ему, ей, им.
The structure благодарен + dative stays the same; only the word in dative changes.

Is it necessary to say я, or can I just say Благодарен другу за помощь?

You can omit я, especially in informal speech or writing, and say:

Благодарен другу за помощь.

It will still be understood as “(I am) grateful to (my) friend for the help,” because:

Благодарен is masculine singular, so the listener will assume the (implicit) subject is he / I (male) or another masculine noun from context.

However:

Using я is very natural and clear: Я благодарен другу за помощь.
Omitting я can sound a bit more elliptical / written / formulaic, like in short notes, announcements, public statements, etc.

Both are grammatically correct; with я is the safe default for learners.

Is the word order fixed? Can I say Я другу благодарен за помощь or За помощь я благодарен другу?

Russian word order is relatively flexible, so several variants are possible, with slightly different emphasis:

Я благодарен другу за помощь.
Neutral, standard: focus on the whole fact “I am grateful to my friend for the help.”
Я другу благодарен за помощь.
Emphasises благодарен, contrastive in some contexts (e.g. “I am grateful to my friend, not angry.”)
Still correct, but less basic for a textbook sentence.
Я благодарен за помощь другу.
Slightly shifts focus: “I am grateful for the help to my friend (specifically him).”
За помощь я благодарен другу.
Puts за помощь first, so the cause (“for the help”) is emphasised.

All of these are grammatical.
For a learner, Я благодарен другу за помощь is the best standard pattern to remember.

How formal or polite is Я благодарен другу за помощь? Is this how people actually talk?

The sentence is neutral and polite, but it sounds slightly bookish / written because of:

the short-form adjective благодарен, and
the use of другу instead of a direct “you”.

In real everyday speech, people often say:

Спасибо за помощь. – Thank you for the help.
Большое спасибо за помощь. – Big thanks for the help.
Спасибо тебе за помощь. / Спасибо вам за помощь.

If you want a spoken full sentence with я благодарен addressed to a person, you could say:

Я очень благодарен тебе за помощь. (male speaker, informal)
Я очень благодарна вам за помощь. (female speaker, polite or plural)

So your sentence is natural, especially in written contexts, in formal speech, or when describing your attitude to a third person:

Я благодарен другу за помощь, которую он мне оказал.
“I am grateful to my friend for the help he gave me.”

What is the difference between Я благодарен другу за помощь and Я благодарю друга за помощь or just Спасибо за помощь?

They are close in meaning but differ in structure and nuance:

Я благодарен другу за помощь.
- Uses the adjective благодарен.
- Describes your state / attitude: “I am (in a state of being) grateful.”
Я благодарю друга за помощь.
- Uses the verb благодарю (“I thank”).
- Focus on the action of thanking itself.
Спасибо за помощь.
- Literally: “Thanks for the help.”
- The most common, simple, everyday way to say “Thank you for the help.”

Roughly:

благодарен = “am grateful” (state)
благодарю = “I thank” (action)
спасибо = “thanks” (fixed expression)

Why is помощь used, not a verb like “helping”? Is помощь countable?

Помощь is a noun meaning “help, assistance”. In Russian, it is typically treated as uncountable in this kind of context, similar to English “help”:

You normally don’t say “a help” in English; you say “for your help.”
Likewise, in Russian: за помощь, not за одну помощь in this meaning.

So:

помощь – help (as a general concept / amount of help)
за помощь – for (the) help (you gave)

If you really want to count separate “acts of help,” Russian tends to rephrase, for example:

за твою помощь – for your help
за помощь, которую ты мне оказал – for the help that you gave me

But in the basic sentence, помощь is just an uncountable noun, accusative singular, after за.

Could I say Я благодарный другу за помощь instead of Я благодарен другу за помощь?

This is not natural and usually feels wrong to a native speaker.

The full-form adjective благодарный is typically used before a noun, as an attribute:
- благодарный ученик – a grateful student
- благодарный слушатель – a grateful listener

When you want to say “I am grateful to X for Y,” Russian strongly prefers the short form:

Я благодарен другу за помощь. – natural
Он благодарен родителям. – natural

Using я благодарный другу sounds like a learner’s mistake.
So: keep благодарный before nouns (благодарный человек) and благодарен in predicate constructions (я благодарен…).