Learn Hardcore German: Der Vorrat ist fast leer, also kaufe ich heute Mehl und Gemüse. - The stock is almost empty, so today I am buying flour and vegetables.

No. This is a classic false friend. German also here means therefore or so, introducing a consequence. For the additive meaning of English also, German uses auch.

Why is it also kaufe ich and not also ich kaufe?

German main clauses are verb-second. When also stands at the start of the clause, it occupies the first position, so the finite verb (kaufe) must come second and the subject (ich) follows: also kaufe ich .... If you omit also, you would say Ich kaufe heute ... with the subject in first position.

Do I need the comma before also?

Yes. You are linking two independent main clauses. With also as a linking adverb, German requires a comma (a semicolon is also possible in more formal writing).

Can I make it two sentences, starting the second with Also?

Yes: Der Vorrat ist fast leer. Also kaufe ich heute Mehl und Gemüse. That is stylistically fine and very common.

What is the difference between kaufen and einkaufen?

kaufen focuses on buying a specific thing and takes a direct object: Ich kaufe Mehl.
einkaufen is the shopping activity. You can say Ich kaufe heute ein (I’m doing the shopping today) or also use it with objects: Ich kaufe Mehl und Gemüse ein. Using plain kaufen with the objects, as in the sentence, is perfectly natural.

Why are there no articles before Mehl and Gemüse?

Both are typically uncountable, generic food nouns. When you mean an unspecified amount, you use no article: Mehl, Gemüse. If you specify a quantity, add a measure word: ein Kilo Mehl, etwas Gemüse, viel Gemüse. Using definite articles (das Mehl, das Gemüse) would point to specific, known items.

What are the genders and plurals?

der Vorrat (plural die Vorräte)
das Mehl (no plural in normal use)
das Gemüse (normally uncountable singular; a plural is rare and means kinds of vegetables, often expressed as Gemüsesorten)

Is the placement of heute correct? Could it go elsewhere?

Yes. The neutral order for adverbials is roughly time–manner–place. ... kaufe ich heute Mehl und Gemüse is natural. You can also front it for emphasis: Heute kaufe ich Mehl und Gemüse. ... kaufe ich Mehl und Gemüse heute is possible but less neutral.

Is Der Vorrat ist fast leer idiomatic? Any alternatives?

Yes, it’s idiomatic. Alternatives include:

Die Vorräte sind fast aufgebraucht.
Der Vorrat geht zur Neige.
Colloquial: Der Vorrat ist fast auf. For consumables, you’ll also hear X ist (fast) alle (for example, Die Milch ist fast alle).

What does fast mean here?

It means almost or nearly. It does not mean quick. For quick, use schnell. A synonym of fast in this sense is beinahe.

Why present tense kaufe instead of a future form?

German commonly uses the present to talk about the near future, especially with a time word like heute. A future form (werde kaufen) is possible but not necessary unless you want to emphasize futurity or intention.

Can I replace also with deshalb, daher, darum, or somit?

Yes. All can introduce a consequence and trigger the same inversion: ... deshalb kaufe ich ..., ... daher kaufe ich ..., ... darum kaufe ich ..., ... somit kaufe ich .... somit is more formal. Do not use German so here to mean therefore.

Is Gemüse countable? How do I say some vegetables or a vegetable?

In everyday German, Gemüse is a mass noun: etwas Gemüse, viel Gemüse, kein Gemüse. To talk about kinds, say Gemüsesorten or verschiedenes Gemüse. Using ein Gemüse is rare in everyday speech; people usually name the item (eine Karotte, eine Tomate). In restaurant language, ein Gemüse can mean a vegetable side dish.

Could I express the cause with weil instead of using also for the result?

Yes. Cause: Weil der Vorrat fast leer ist, kaufe ich heute Mehl und Gemüse. Result: Der Vorrat ist fast leer, also kaufe ich ... Both are natural. weil is a subordinating conjunction, so the verb goes to the end in the weil clause (... leer ist).

Which case are Mehl and Gemüse in?

Accusative. They are the direct objects of kaufen. With neuter mass nouns you won’t see a case ending, but if you used definite articles, you would get Ich kaufe heute das Mehl und das Gemüse.