Перед анализом крови лучше не пить сладкий чай.

Because the preposition перед normally takes the instrumental case when it means before in time or in front of in space.

So:

• анализ = nominative
• анализом = instrumental singular

In this sentence, перед анализом крови literally means before the blood analysis / before the blood test.

Крови is in the genitive singular because it depends on the noun анализ.

Russian often uses this pattern:

• анализ чего? = analysis of what?
• анализ крови = analysis of blood

So the phrase is built like this:

• анализ = analysis
• крови = of blood

This is a very common noun + genitive structure in Russian.

Yes. Even though the literal meaning is analysis of blood, in normal English this is often translated as blood test.

So:

• сдать анализ крови = to have a blood test / to give blood for testing
• перед анализом крови = before a blood test

This is a very natural Russian expression.

Here лучше means it is better.

Russian often uses лучше + infinitive to give advice in an impersonal way:

• лучше подождать = it is better to wait
• лучше не есть = it is better not to eat
• лучше не пить сладкий чай = it is better not to drink sweet tea

There is no explicit subject like you. Russian often leaves that unstated when giving general advice.

So the sentence means something like:

• Before a blood test, it is better not to drink sweet tea.

Because Russian often leaves out the subject when it is general, obvious, or not important.

This sentence gives general advice, so Russian does not need to say вам or вы.

Compare:

• Перед анализом крови лучше не пить сладкий чай.
  = Before a blood test, it is better not to drink sweet tea.

You could make it more personal, for example:

• Перед анализом крови вам лучше не пить сладкий чай.
  = Before a blood test, you had better not drink sweet tea.

But the version without вам is very natural and common.

Пить is imperfective and is the natural choice here because the sentence gives general advice about the activity of drinking sweet tea.

• пить = to drink, in a general or ongoing sense
• выпить = to drink up / to have a drink, often seen as one completed act

In advice and prohibitions, Russian often uses the imperfective:

• лучше не есть
• нельзя курить
• не стоит спешить

So лучше не пить сладкий чай means it is better not to drink sweet tea at all before the test.

Using выпить would sound more like not to have one specific serving, which is less neutral here.

Here сладкий чай is the direct object of пить, so it is in the accusative case.

For inanimate masculine singular nouns, the accusative looks the same as the nominative:

• nominative: сладкий чай
• accusative: сладкий чай

So the form does not change.

You may have seen that negation sometimes allows or causes the genitive in Russian, but in modern standard Russian, with a concrete direct object like this, accusative is the most normal choice:

• не пить сладкий чай = standard and natural

In this context, сладкий чай most naturally means sweetened tea, usually tea with sugar or something similar added.

So it is not mainly describing a naturally sweet type of tea. It usually means ordinary tea that is made sweet.

That is why the sentence is medical/dietary advice: sugar can affect test results.

Yes, you can, but there is a nuance.

• перед анализом крови = before the blood test, often with a sense of shortly before
• до анализа крови = before the blood test / prior to the blood test

Both can work, but перед often sounds a bit more like in the time right before something, while до can be a little broader.

So:

• Перед анализом крови лучше не пить сладкий чай.
  = very natural
• До анализа крови лучше не пить сладкий чай.
  = also possible

Russian word order is flexible, and the beginning of the sentence often contains the context or time setting.

By starting with Перед анализом крови, the speaker first sets the situation:

• Before a blood test...

Then comes the advice:

• ...лучше не пить сладкий чай.

This order is very natural because it tells you when the advice applies before giving the advice itself.

Other word orders are possible, for example:

• Лучше не пить сладкий чай перед анализом крови.

This also sounds natural, but the original version emphasizes the time condition first.

Yes, there is a difference.

• лучше не пить = it is better not to drink; advice, recommendation
• нельзя пить = you must not drink / it is not allowed to drink; much stronger

So this sentence sounds like medical advice or a practical recommendation, not necessarily an absolute prohibition.

If someone wanted to sound stricter, they might say:

• Перед анализом крови нельзя пить сладкий чай.

But лучше не пить is softer and often sounds more natural in everyday explanation.

Grammatically, the sentence is specifically about sweet tea.

But in real-life context, the underlying idea is usually about avoiding sugar before a blood test. Russian often gives advice through a concrete example, especially if that example is common in daily life.

So the sentence literally talks about tea, but the practical meaning may be broader depending on the situation.