Одного дня недостаточно.

Because достаточно and недостаточно normally require the thing being measured to appear in the genitive case.

So Russian structures this more like:

• not enough of one day

rather than the English-style subject + verb pattern one day is not enough.

That is why both words change:

• один → одного
• день → дня

You see the same pattern in other sentences:

• Этого недостаточно.
• Времени недостаточно.
• Одного часа недостаточно.

Один declines like an adjective and must agree with the noun it modifies in gender, number, and case.

Here the noun is:

• день — masculine singular

And because the whole phrase is in the genitive singular, один also has to be genitive singular masculine:

• nominative: один день
• genitive: одного дня

So одного is simply the correct genitive form of один here.

In the present tense, Russian usually leaves out the verb to be.

So where English says:

• One day is not enough

Russian just says:

• Одного дня недостаточно.

There is still a complete sentence, but the present-tense is is understood.

If you put it in the past or future, a form of быть appears:

• Одного дня было недостаточно.
• Одного дня будет недостаточно.

In this sentence, недостаточно functions as a predicative word: it is the main predicate of the sentence and means not enough / insufficient.

Different grammar sources may label it a little differently, but the important thing for a learner is this:

• it can stand as the main idea of the sentence
• it often appears in an impersonal construction
• the thing that is enough or not enough is commonly put in the genitive

So in Одного дня недостаточно, недостаточно is doing the job that is not enough does in English.

Yes, you can. It is grammatical, but it feels different.

• Одного дня недостаточно — the more common, natural everyday pattern
• Один день недостаточен — more formal, bookish, or analytical

The difference is structural too:

• Одного дня недостаточно uses the genitive construction with недостаточно
• Один день недостаточен uses недостаточен, a short-form adjective, with один день as the nominative subject

Both are correct, but the version in your sentence is the one learners will hear more often in normal speech.

Yes. Russian word order is flexible.

You can also say:

• Недостаточно одного дня.

Both sentences are correct. The difference is mostly about focus and emphasis.

• Одного дня недостаточно — puts early focus on one day
• Недостаточно одного дня — starts with the idea not enough

In many situations, both sound natural.

This is a very common question.

In adjective/pronoun-type endings like -ого and -его, the letter г is traditionally pronounced like в in standard Russian pronunciation.

So:

• одного is pronounced roughly adnaVO
• моего sounds like mayeVO
• большого sounds like bal'shaVO

This is normal and something you just learn as part of the pattern. The spelling stays -ого / -его, but the pronunciation is usually -ово / -ево.

In the neutral meaning not enough / insufficiently, it is normally written as one word:

• недостаточно

That is the standard spelling in a sentence like:

• Одного дня недостаточно.

You may see не достаточно written separately only when не is being strongly emphasized as a separate negation, usually with contrast or special wording.

So for your sentence, the normal spelling is definitely:

• Одного дня недостаточно.

Not really. It is a normal declension pattern.

День is a masculine noun ending in -ь, and its genitive singular is:

• день → дня

So the soft sign disappears and the case ending changes.

A few useful forms are:

• nominative: день
• genitive: дня
• dative: дню
• instrumental: днём
• prepositional: дне

So дня here is just the regular genitive singular form required by недостаточно.