mada keeki wo tabete imasen.

まだ means “still” or “not yet.” When you want to express “haven’t done something yet,” you typically place まだ immediately before the verb phrase or at the very front of the sentence. For example:

• Natural: まだケーキを食べていません。
• Less common: ケーキをまだ食べていません。

Putting まだ too far from the verb can make the sentence sound awkward or less natural.

食べていません is the negative form of the ～ている aspect, which here expresses that the action has not occurred up to now (a perfect or resultant state).

• まだケーキを食べていません。 = “I haven’t eaten the cake yet.”

On the other hand, 食べません is the simple non-past negative (“I won’t eat” or “I don’t eat [habitually]”), so it’s not used to mean “have not eaten yet.”

The ～ている form can express:
• Progressive (“is doing”)
• Perfect/resultant state (“has done”)

In combination with まだ and the negative, まだ食べていません indicates the perfect aspect: the cake remains uneaten up to now. So 食べていません here means “have not eaten.”

Yes. Changing を to は makes ケーキ the topic rather than just the object. This adds contrast or emphasis on the cake:

• ケーキはまだ食べていません。
  (As for the cake, I haven’t eaten it yet.)
  Using を is more neutral: you’re simply stating the action hasn’t happened.

Definitely. The plain (informal) negative is 食べていない, while the polite form is 食べていません. The meaning stays the same (“haven’t eaten yet”), but you adjust formality to suit your audience:

• Polite: まだケーキを食べていません。
• Plain: まだケーキを食べていない。