Без чеснока вкус был бы слабее, поэтому не стоит забывать про специи.

The preposition без (without) always requires the genitive case.
So чеснок → чеснока (genitive singular): без чеснока = without garlic.

был бы is the conditional / hypothetical form: would be.
Russian often forms this with:

• a past-tense form of быть (был/была/было/были) +
• the particle бы

So:

• вкус был бы слабее = the taste would be weaker (if garlic weren’t there).

Because вкус is a masculine noun, so past tense agrees with it:

• вкус был (masc.)
• еда была (fem.)
• пиво было (neut.)
• специи были (plural)

Then you add бы for the conditional: вкус был бы…

Russian often omits если when the conditional meaning is already clear, especially with без + genitive and бы.
Без чеснока вкус был бы слабее is understood as: If there were no garlic, the taste would be weaker.

You can also say:

• Если бы не было чеснока, вкус был бы слабее. (more explicit)

слабее is the comparative form of the adjective слабый (weak):

• слабый = weak
• слабее = weaker

Russian comparatives often appear in this short form ending in -ее / -ей.

Because поэтому (therefore/so) introduces a second clause, and Russian typically separates clauses with a comma:

• …вкус был бы слабее, поэтому…

This is standard punctuation for two related clauses.

поэтому means therefore / so / that’s why, not because.
It points to a result, not a cause.

Cause = потому что (because):

• …потому что… = because…

Result = поэтому (therefore):

• …, поэтому … = …, so …

не стоит + infinitive is an impersonal construction meaning it’s not worth / one shouldn’t.

Literally, стоить means to be worth.
So:

• не стоит забывать = it’s not worth forgetting → natural English: you shouldn’t forget

No subject is required; it’s a general recommendation.

With не стоит, Russian commonly uses the imperfective infinitive to mean “don’t (habitually / in general) do this”:

• не стоит забывать = you shouldn’t forget (as a general rule)

не стоит забыть is unusual here because perfective would imply a single completed act (“it’s not worth forgetting once”), which doesn’t fit the general advice.

Both can mean “about,” but they differ in style and nuance:

• про + accusative (про специи) is more conversational and often means “don’t forget about / don’t forget to include.”
• о + prepositional (о специях) is more neutral/formal and often fits “talk/think about.”

Here, не стоит забывать про специи is very natural: “don’t forget about the spices (i.e., to use them).”

про requires the accusative case.
специи is plural, inanimate, so:

• nominative plural = специи
• accusative plural = специи (same form)

So the spelling doesn’t change, but the case role does.

Yes—Russian word order is flexible, but emphasis changes.

For example:

• Поэтому не стоит забывать про специи: без чеснока вкус был бы слабее.
  Puts the conclusion first, then explains why.

The original order feels natural: reason → result/advice.