В пакете было только пятьсот граммов муки, поэтому мне пришлось купить ещё один килограмм.

Because в can take different cases depending on meaning:

• в + accusative = motion into something
  
  • в пакет = into the bag
• в + prepositional = location inside something
  
  • в пакете = in the bag

Here the sentence describes where the flour was, not movement, so Russian uses the prepositional case: в пакете.

In this sentence, было is the usual neuter singular past form used in an existential-style construction: В пакете было... = There was/there were in the bag...

With amounts, quantities, and especially mass nouns like мука, Russian very often uses singular было:

• было много воды
• было пять килограммов сахара
• было только пятьсот граммов муки

So even though the idea involves grams, the whole phrase is treated as a quantity/amount, and было sounds natural.

Пятьсот means five hundred. In this sentence it is in the nominative/accusative form.

Russian numerals do decline, including пятьсот. For example:

• nominative/accusative: пятьсот
• genitive: пятисот
• dative: пятистам
• instrumental: пятьюстами
• prepositional: о пятистах

So yes, it changes, but here the basic form пятьсот is the correct one.

After numerals ending in 5–9, 0, and after the teens 11–14, the counted noun usually goes in the genitive plural.

That is why:

• пять граммов
• десять граммов
• пятьсот граммов

Compare:

• один грамм
• два грамма
• три грамма
• четыре грамма
• пять граммов

This is one of the standard Russian number patterns.

Because after a measure word like грамм, килограмм, литр, чашка, Russian usually puts the substance being measured in the genitive:

• килограмм сахара
• литр молока
• чашка чая
• пятьсот граммов муки

Here муки is the genitive singular of мука. The idea is literally something like five hundred grams of flour.

Только means only, and here it limits the amount:

• было только пятьсот граммов муки = there were only 500 grams of flour

Putting только before the quantity is the most natural way to show that the amount was small or insufficient.

Russian word order is flexible, but changing the position of только can slightly change what is being emphasized.

Поэтому means therefore, so, or that’s why. It introduces the result of the first clause:

• В пакете было только пятьсот граммов муки, поэтому...
• There were only 500 grams of flour in the bag, so...

The comma is needed because there are two clauses:

1. В пакете было только пятьсот граммов муки
2. мне пришлось купить ещё один килограмм

Russian normally separates them with a comma here.

Мне пришлось comes from the verb прийтись and means I had to in the sense of circumstances made it necessary.

So:

• мне пришлось купить = I had to buy
• literally, it is an impersonal construction with the person in the dative

This is a little different from я должен был, which often sounds more like I was supposed to / I was obliged to.

Compare:

• Мне пришлось уйти рано = I had to leave early, because circumstances required it
• Я должен был уйти рано = I was supposed to leave early / I was under an obligation to leave early

In your sentence, мне пришлось fits well because the shortage of flour created the necessity.

Because пришлось is used in an impersonal construction, and the person affected by the necessity goes in the dative:

• мне пришлось = I had to
• тебе пришлось = you had to
• ему пришлось = he had to

This pattern is very common in Russian:

• мне нужно
• ему надо
• нам пришлось

So мне is not the subject in the nominative; it is the experiencer in the dative.

Because the sentence refers to a single completed action: buying one more kilogram.

• купить = to buy, as a completed act
• покупать = to buy, as a process, repeated action, or without focus on completion

Here the meaning is:

• there wasn’t enough flour
• as a result, I had to go and complete the purchase

So купить is the natural choice.

Because after купить, the direct object is in the accusative case, and килограмм is an inanimate masculine noun.

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

• nominative: один килограмм
• accusative: один килограмм

So:

• купить ещё один килограмм is correct

The form одного килограмма would not be right here.

Ещё here means more or another.

So:

• один килограмм = one kilogram
• ещё один килограмм = one more kilogram / another kilogram

It shows that the speaker already had some flour, but needed an additional amount.

It can be repeated, but Russian often leaves it out when the meaning is already obvious from context.

Full version:

• ...мне пришлось купить ещё один килограмм муки

Shorter version:

• ...мне пришлось купить ещё один килограмм

Since the sentence has already mentioned мука, Russian naturally omits the repeated noun. English does this too sometimes: I had to buy another kilogram.