Learn Hardcore Ukrainian: Я помітив якусь помилку в адресі, бо літера пропущена. - I noticed some mistake in the address because a letter is missing.

помітив is past tense masculine singular of the perfective verb помітити (to notice once, completed action). It implies a single, completed act of noticing.
If the speaker is female, use помітила; if plural, помітили.
The imperfective counterpart is помічати (past: помічав/помічала), used for habitual or ongoing actions (e.g., I used to notice).

What does якусь mean here, and why not якась?

якусь = feminine accusative of the indefinite pronoun якийсь (some, some kind of). It agrees with the direct object помилку (accusative).
якась is feminine nominative and would be used if the noun were the subject: Якась помилка в адресі…
Note the soft sign: the postfix is -сь (not an apostrophe): якийсь, якась, якусь.

Why is it помилку (with -у) and not помилка?

Because помилку is the accusative singular of the feminine noun помилка. As the direct object of a transitive verb (помітив), it takes the accusative.

Why в адресі and not у адресі?

Both в and у mean in/at; Ukrainian alternates them for euphony. Between two vowels, в is preferred. Here we have …помилку (vowel -у) в адресі (vowel а-), so в is smoother. У адресі isn’t wrong, just less euphonic in this spot.

What case is адресі, and is адреса the right Ukrainian word?

адресі is the locative (місцевий) singular after в/у meaning in: в адресі = in the address.
The correct Ukrainian noun is адреса (feminine). Some learners mistakenly use адрес (a Russism) as a singular; in Ukrainian, адрес is only the genitive plural of адреси.

Should there be a comma before бо?

Yes. A comma is required before бо (because) introducing the reason clause: …, бо …. The same applies to тому що, оскільки, etc.

How do бо, тому що, and оскільки differ?

All mean because:

бо: short, very common in speech and neutral writing.
тому що: neutral and very common; slightly more formal than бо.
оскільки: since/as; more formal or bookish. Avoid calques like так як for because in standard Ukrainian.

What is пропущена grammatically?

It’s a past passive participle used adjectivally: пропущений/пропущена/пропущене/пропущені. It agrees with літера (feminine singular nominative): літера пропущена = the letter is omitted/missing.

Could I say бо пропущено літеру instead? What’s the difference?

Yes. Пропущено літеру is an impersonal -но/-то construction that’s very natural when the doer is irrelevant.

літера пропущена emphasizes the state/result (the letter is missing).
пропущено літеру emphasizes the action done (a letter has been omitted).

Are there other natural ways to say a letter is missing?

Yes:

бо бракує літери
бо відсутня одна літера
бо пропущена одна літера
All are idiomatic; adding одна highlights the number.

Can I drop якусь and just say Я помітив помилку…?

Absolutely. Ukrainian has no articles, so помилку is already indefinite in context. Якусь adds a nuance of vagueness (some sort of), but it’s optional.

Is літера the same as буква?

They’re near-synonyms meaning letter (of the alphabet). Літера is slightly more formal/technical; буква is very common in everyday speech. Both work here.

Is на адресі ever correct? How do I say at this address?

Use:

за цією адресою = at this address (set phrase, common in official style).
за адресою … = at the address …
На адресі is generally not used for this meaning. For a mistake inside the address text, в адресі is the right choice.

Any quick agreement reminders from this sentence?

Past-tense verbs agree with the subject’s gender/number: я помітив/помітила/ми помітили.
Participles/adjectives agree with the noun: літера пропущена, буква пропущена; plural: літери пропущені.