После работы я устал, но доволен днём.

После always takes the genitive case.

• The noun работа (work) in the genitive singular is работы.
• So после работы literally means after (the) work.

Pattern to remember:

• после чего? – after what? → genitive
  
  • после урока
  • после встречи
  • после войны

Устал is:

• the past tense, masculine singular form of the verb устать (to get tired),
• which is a perfective verb (focus on result: became tired / ended up tired).

In Russian past tense, you usually don’t use a separate word for “was” with such forms:

• я устал = I got tired / I am tired (as a result)
• Literally: I-tired (masc., past)

The idea of “was” is built into the past form устал, so you don't say something like я был устал in normal speech.

The past tense in Russian agrees with gender and number:

• masculine singular: я устал – I (man) got tired
• feminine singular: я устала – I (woman) got tired
• neuter singular (for it): оно устало
• plural (any mixed or all-female/male group): мы устали – we got tired

So the sentence for a woman would be:

• После работы я устала, но доволена днём. (see доволена note below)

Yes, there is both a grammatical and a stylistic difference.

• я устал

  • verb (past tense of устать)
  • main everyday way to say I’m tired / I got tired
  • describes your state as a result of something (here: work)

• я усталый

  • full-form adjective (усталый = tired) used as a predicate
  • sounds more descriptive, often a bit more literary or emotional
  • closer to I am a tired person or I am in a tired state

In normal conversation, people almost always say я устал / я устала, not я усталый.

Доволен is a short-form adjective. Russian has many adjectives that have:

• a full form: довольный
• a short form: доволен / довольна / довольно / довольны

In this sentence:

• я доволен = I am satisfied / content
• доволен is the usual predicative form (used after to be, even when to be is not written).

The full form довольный is more often:

• attributive: довольный клиент – a satisfied customer
• or gives a slightly different nuance: я довольный can sound like I’m a satisfied guy, more character-like or emotional.

So in your sentence, доволен is the natural, standard choice.

Днём is:

• the instrumental singular of день (day).

Many adjectives (and some verbs) in Russian require the instrumental case after them.
Доволен takes the instrumental:

• доволен кем? чем? – satisfied with whom? with what? → instrumental

So:

• доволен днём = satisfied with the day
  
  • днём answers чем? – with what? → instrumental.

Yes, you can, and both are grammatical.

• доволен этим днём – satisfied with this day

  • Emphasizes this particular day, maybe contrasting it with other days.

• доволен днём работы – satisfied with the working day / with the day of work

  • Slightly more formal; focuses on the day as a work day.

The original доволен днём is more neutral and general: happy with how the day went.

In Russian, you often omit the subject in the second (and later) clauses when it’s the same as in the first clause and context is clear.

• Full form would be: После работы я устал, но я доволен днём.
• Natural speech: После работы я устал, но доволен днём.

Russian relies heavily on verb endings and context to show the subject, so repeating я would sound more heavy and less elegant here, though it’s not wrong.

Но is a coordinating conjunction meaning but. In Russian, when но connects two clauses (each with its own predicate), you must put a comma before it:

• После работы я устал, но (я) доволен днём.
  
  • Clause 1: я устал
  • Clause 2: (я) доволен днём

Since there are two separate statements contrasted with но, a comma is required.

You could say:

• После работы я устал, а доволен днём.

but it sounds less natural and a bit stylistically off in most contexts.

Nuance:

• но = but, however – shows a clear, often stronger contrast
  
  • tired vs. satisfied → strong contrast → но fits perfectly.
• а = often and / whereas / but – weaker contrast, often just a comparison or shift
  
  • more like on the other hand or while.

Here, being tired yet satisfied is felt as a real contrast, so native speakers strongly prefer но.

Yes, Russian word order is quite flexible. All of these are possible:

• После работы я устал, но доволен днём.
• Я устал после работы, но доволен днём.
• Я устал, но после работы доволен днём. (less natural; sounds like after work I’m satisfied with the day, slightly odd)

Most natural are:

• После работы я устал, но доволен днём. (focus on “after work”)
• Я устал после работы, но доволен днём. (focus on “I got tired”)

The original version is very typical and sounds smooth.

Orthographically:

• днём and днем are both accepted spellings in modern Russian.
• In many printed texts, ё is often replaced by е, so you’ll see днем a lot.

Pronunciation is always with [о] under stress:

• днём = [днём] (roughly dnyom in English letters), one syllable.

Using ё is often clearer for learners because it explicitly shows:

• the stress, and
• that it’s pronounced /o/, not /e/.

Stress pattern:

• уста́л – stress on the second syllable
• дово́лен – stress on the second syllable
• рабо́ты – stress on бо (same as рабо́та)
• днём – only one syllable, so the stress is on ём

So spoken: После рабо́ты я уста́л, но дово́лен днём.