Learn Hardcore Japanese: syoutaimeeru ga mada todoite imasen. - The invitation email hasn't arrived yet.

Because 届く is an intransitive verb meaning “to arrive,” and the thing that arrives is the grammatical subject, marked by が. Using を would be ungrammatical with 届く. If you switch to a transitive verb like 受け取る (to receive), then you use を: 招待メールをまだ受け取っていません.

Can I use は instead of が here?

Yes. 招待メールはまだ届いていません is also natural.

が often presents the subject as new/neutral information and is common with intransitive verbs like 届く.
は makes 招待メール the topic and can imply contrast or set the frame of discussion. For example: 招待メールはまだ届いていませんが、案内メールは届きました。

What does 届いていません mean exactly? Why use the ている form?

届いていません is the negative of 届いています (the ている form). With many intransitive verbs, 〜ている describes a current result/state, not a continuous action. So 届いていません means “it is in the state of not having arrived (yet).” It’s like “hasn’t arrived yet,” not “is not arriving.”

Why is it いません and not ありません if an email is inanimate?

The いる in 〜ている is an auxiliary that expresses aspect (state/progress), not existence. This auxiliary is always いる/います/いません, regardless of whether the subject is animate or inanimate. Do not say 届いてありません.

Can I say まだ届きません instead of まだ届いていません?

You can, but the nuance changes:

まだ届いていません = current status/result: “It hasn’t arrived yet” (most natural for updates).
まだ届きません = non-past negative; can sound predictive/generic (“it won’t arrive yet/doesn’t arrive yet”). It’s used but less typical for a simple status update.

How does まだ work? How is it different from もう?

まだ + negative = “not yet”: まだ届いていません.
まだ + affirmative = “still”: まだ時間があります (There’s still time).
もう + affirmative = “already”: もう届きました (It has already arrived).
もう + negative = “no longer”: もう届きません (It no longer arrives/is no longer delivered), e.g., after unsubscribing.

Where should まだ go in the sentence? Is word order flexible?

All of these are natural, with slight differences in focus:

まだ招待メールが届いていません
招待メールはまだ届いていません
招待メールがまだ届いていません (your version) Avoid putting まだ at the very end; it normally precedes the predicate it modifies.

Can I just say まだ届いていません without mentioning 招待メール?

Yes. Japanese often omits the subject when it’s clear from context. If needed, you can refer back with それ: それはまだ届いていません.

Is 招待メール the most natural way to say “invitation email”? What about 招待のメール?

Both are fine.

招待メール is a common, concise compound and feels set-phrase-like in tech/business contexts.
招待のメール is also correct and a bit more descriptive. For a physical mailed invitation card, use 招待状.

What’s the difference between 届く, 届ける, 送る, and 受け取る?

届く (intransitive): something arrives to someone (〜に). Example: 招待メールが私に届く.
届ける (transitive): someone delivers something to someone. 招待メールを届ける.
送る (transitive): someone sends something. 招待メールを送る.
受け取る (transitive): someone receives something. 招待メールを受け取る. Rephrasings:
“It hasn’t arrived yet”: 招待メールがまだ届いていません.
“They haven’t sent it yet”: 招待メールをまだ送っていません.
“I haven’t received it yet”: 招待メールをまだ受け取っていません.

Can I use 来る and say メールがまだ来ていません?

Yes. メールがまだ来ていません is very common in conversation and means the same thing. 届く can sound a bit more neutral/formal/technical; 来る is more colloquial.

How do I adjust the politeness level?

Plain: 招待メールがまだ届いていない
Polite: 招待メールがまだ届いていません (your sentence)
Extra-polite/business: 招待メールがまだ届いておりません
Casual speech contractions: まだ届いてない, まだ届いてません, まだ届いてないです (last one mixes plain/polite; common in speech)

How do I indicate “to me/us” explicitly?

Mark the recipient with 〜に(は):

私にはまだ招待メールが届いていません (It hasn’t arrived to me yet).
Business: 弊社にはまだ招待メールが届いておりません (It hasn’t arrived to our company yet).

If I want to say “It hasn’t been sent yet,” how do I say that? Does this sentence imply that?

Your sentence only states “hasn’t arrived,” not whether it was sent. To say “hasn’t been sent”:

Passive/unknown sender: 招待メールはまだ送られていません.
Active/known agent: 担当者は招待メールをまだ送っていません.

Are the spaces in the sentence normal?

No. Standard Japanese doesn’t use spaces. Write it as 招待メールがまだ届いていません。 The spaces are often added in teaching materials.

How do I read/pronounce the sentence?

招待メール: しょうたいメール (shōtai mēru)
届いていません: とどいていません (todoite imasen)
Full: しょうたいメールがまだとどいていません / shōtai mēru ga mada todoite imasen

Can I drop が after 招待メール?

In casual speech, yes, by making it a loose topic: 招待メール、まだ届いていません. In careful writing, prefer が or switch to は.

How would I answer or follow up in conversation?

Q: 招待メールは届きましたか。 A: まだです。 or まだ届いていません。
Polite follow-up request: お手数ですが、再送していただけますか。
More formal: お手数ですが、再送いただけますと幸いです。

Is まだ the same as また?

No.

まだ = still/yet: まだ届いていません (not yet), まだあります (still have).
また = again/also: また連絡します (I’ll contact you again). They are not interchangeable.