E-mail je stigao davno.

It is the standard Croatian past tense (often called the perfect).

It is built with:

• je — the 3rd person singular of biti (to be)
• stigao — the past participle of stići (to arrive / reach)

So je stigao can correspond to English arrived or has arrived. In this sentence, because of davno (long ago), English normally uses arrived: The email arrived long ago.

Because je is a clitic: a short, unstressed word that usually goes in second position in the clause.

So Croatian prefers:

• E-mail je stigao davno.

Not the neutral:

• E-mail stigao je davno.

This second-position rule is very common in Croatian with words like je, sam, si, smo, ste, su.

The past participle agrees with the subject in gender and number.

Here the subject is e-mail, which is treated as masculine singular, so the verb form is:

• stigao = masculine singular

Compare:

• Poruka je stigla. — The message arrived. (poruka is feminine)
• Pismo je stiglo. — The letter arrived. (pismo is neuter)

Yes. In everyday Croatian, e-mail is usually treated as a masculine noun.

So you get forms like:

• e-mail — nominative
• e-maila — genitive
• e-mailu — dative/locative
• e-mailom — instrumental

That is why the sentence has stigao, not stigla.

Stići basically means to arrive, to reach, or to get to.

In this sentence, it means the email arrived. It focuses on the completed event: the email reached you / your inbox.

It is also a perfective verb, so it naturally fits a completed action.

A related imperfective verb is stizati, used for repeated or ongoing arrival:

• E-mailovi su stalno stizali. — Emails kept arriving.

Davno means long ago or a long time ago.

It is an adverb, so it does not change form.

In this sentence:

• E-mail je stigao davno. = The email arrived long ago.

It tells you when the action happened.

Yes. Croatian word order is fairly flexible, and davno can move.

For example:

• E-mail je stigao davno. — neutral
• E-mail je davno stigao. — also natural, with a bit more emphasis on davno
• Davno je stigao e-mail. — more marked, stronger emphasis on long ago

So the meaning stays similar, but the focus changes.

Because Croatian has no articles.

So e-mail can mean:

• the email
• an email

Which one is meant depends on the context.

That is very normal in Croatian, and learners from English often notice it right away.

Yes, absolutely.

• Stigao je e-mail. is a very natural Croatian sentence.
• It often sounds like you are announcing or introducing the event: An email arrived / The email has arrived.

By contrast:

• E-mail je stigao. makes e-mail more like the topic, something already known in the conversation.

So both are correct, but they sound slightly different in terms of information focus.

Not exactly.

• davno = long ago
• odavno = long ago already / for a long time already, often with a sense of already or since long ago

So:

• E-mail je stigao davno. — simple The email arrived long ago.
• E-mail je odavno stigao. — more like The email arrived long ago already / The email has been here for a long time already

In many contexts they are close, but they are not always interchangeable.

No. You may also see or hear:

• email
• mejl — informal, speech-like spelling
• e-poruka — more native/explicit for email message
• e-pošta — often used for email as a system or medium, though in everyday use it may also mean a message

In ordinary conversation, e-mail is very common and fully understandable. In this sentence, it sounds natural.