Learn Hardcore Russian: Мой муж сейчас в офисе и пишет отчёт. - My husband is now in the office and is writing a report.

Russian usually omits the verb “to be” (быть) in the present tense when it simply links a subject and a place/description.
So Мой муж сейчас в офисе literally is “My husband now in (the) office”, but it means “My husband is in the office now.”
You only see forms of быть in the past (был, была, было, были) and future (будет, будут, etc.), or in special emphatic/grammatical constructions.

Why is it мой муж and not something like меня муж?

Мой is the possessive pronoun “my” in the nominative masculine singular, matching муж (husband), which is also nominative masculine singular.
Меня is the genitive/accusative form of я (I/me) and cannot be used as a possessive here.
So мой муж = my husband (subject of the sentence).

Could it be моего мужа instead of мой муж?

Моего мужа is the genitive or accusative form and is used when “my husband” is not the subject, but an object or after certain prepositions.
Here, муж is the subject (the one doing the action), so it must be in the nominative: мой муж.
Example contrast: Я жду моего мужа – I am waiting for my husband (object, accusative).

Is there any difference between муж, мужчина, and парень?

Yes:

муж = husband (marital status, specifically someone’s spouse).
мужчина = man (adult male, neutral, like “a man”).
парень = guy / boyfriend (younger/more informal; can mean just “guy” or romantic partner depending on context).
In Мой муж, it has to be муж because you mean my husband, not just a man or my guy.

Why is it сейчас мой муж в офисе vs Мой муж сейчас в офисе? Is the word order fixed?

Both Мой муж сейчас в офисе and Сейчас мой муж в офисе are correct.
Russian word order is relatively flexible and is used to highlight what’s important.

Мой муж сейчас в офисе is neutral: My husband is in the office now.
Сейчас мой муж в офисе slightly emphasizes “now” (contrasting with some other time or place).

Why is it в офисе and not в офис?

With в, Russian uses different cases depending on meaning:

в
- prepositional (где? – where?) for location: в офисе = in the office.
в
- accusative (куда? – where to?) for motion/direction: в офис = into the office / to the office.
  Here, he is located there, not going there, so it’s в офисе (prepositional).

Why does офис change to офисе?

Офис is a masculine noun. In the prepositional case (used after в for location), masculine nouns ending in a consonant usually take -е: офис → в офисе.
So the change just marks the prepositional case, meaning “in the office”.

Why is there no он before пишет? Why not …и он пишет отчёт?

Russian often omits a repeated subject if it’s clear that it’s the same.
The subject мой муж applies to both verbs: (мой муж) сейчас в офисе и (мой муж) пишет отчёт.
Adding он (…и он пишет отчёт) is also grammatically correct; it just sounds a bit more explicit or slightly more emphatic.

Does пишет mean “is writing” or “writes”?

Both, depending on context. Russian present tense of an imperfective verb like писать covers both English “(he) writes” and “(he) is writing.”
Here, because of сейчас (“now”), the natural English translation is “is writing”:
Он сейчас пишет отчёт = He is writing a report now.

Why is the imperfective писать used and not the perfective написать?

Писать (imperfective) focuses on the process or repeated action: он пишет отчёт = he is in the middle of writing a report.
Написать (perfective) focuses on the result/completion: он напишет отчёт = he will write/finish the report (future), or он написал отчёт = he wrote/has written the report (past, completed).
Since we’re talking about what he is doing now, as an ongoing action, the imperfective пишет is correct.

How is пишет formed from писать?

The infinitive is писать (to write).
In the present tense (imperfective), it conjugates like this:

я пишу – I write / am writing
ты пишешь – you write / are writing
он/она/оно пишет – he/she/it writes / is writing
мы пишем – we write / are writing
вы пишете – you (pl/formal) write / are writing
они пишут – they write / are writing
So он пишет = he writes / is writing.

What case is отчёт in, and why does it look like the dictionary form?

The dictionary form отчёт is nominative singular.
In the sentence, он пишет отчёт, отчёт is a direct object in the accusative case, masculine inanimate.
For masculine inanimate nouns, accusative singular = nominative singular, so отчёт doesn’t change its form.

How is отчёт pronounced, and do Russians always write ё?

Отчёт is pronounced roughly [ат-чёт], with stress on -чёт, and ё sounds like “yo” in “yoga”, but shorter: ot‑TYOT.
In everyday writing, many Russians replace ё with е, so you’ll often see отчет instead of отчёт.
However, the pronunciation still uses ё; stress and dictionaries tell you it’s отчёт.