Learn Hardcore German: Heute esse ich in der Kantine eine Suppe mit Tomaten und Gurken. - Today I eat soup with tomatoes and cucumbers in the canteen.

German main clauses follow the “V2 rule”: the conjugated verb must be the second element in the sentence. If you start with something other than the subject (here Heute), the verb comes next, and then the subject. So the structure is:

Heute (time adverbial)
esse (finite verb)
ich (subject)

If you began with Ich, you’d say Ich esse heute…, but putting Heute first simply pushes ich to third position.

Why is in der Kantine used here, and why der instead of die?

The preposition in has two uses:

With dative, it expresses location (“in” = inside).
With accusative, it expresses movement (“into”).

Since you’re saying where you eat (location), you use the dative. Kantine is feminine (die Kantine), so its dative singular form is der Kantine. Hence in der Kantine.

Why is it eine Suppe and how do we know it’s in the accusative case?

Suppe is a feminine noun (nominative die Suppe). In this sentence, it’s the direct object of esse (the thing being eaten), so it must take the accusative case. The accusative feminine singular of the indefinite article eine remains eine. Thus eine Suppe.

Why aren’t there articles before Tomaten und Gurken?

German does not have an indefinite article in the plural. If you mean “some tomatoes and cucumbers,” you simply omit the article. In effect, Tomaten und Gurken here means “(some) tomatoes and cucumbers.”

Why is the preposition mit used here, and which case does it govern?

mit means “with” when you list ingredients or companions. It always takes the dative case. Since Tomaten and Gurken are plural and already carry the dative plural ending -n, there’s no visible change beyond omitting any article.

Why is mit Tomaten und Gurken placed after eine Suppe rather than directly after esse?

That mit-phrase describes which kind of soup you’re eating—it’s part of the noun phrase eine Suppe mit Tomaten und Gurken. As a rule, modifiers that belong to a noun stay attached to it. If you moved it elsewhere, it could sound like you’re eating in the canteen together with tomatoes and cucumbers.

Why do we use the simple present (esse) instead of something like “I am eating”?

German has no distinct present-continuous tense. The simple present (ich esse) covers both “I eat” and “I am eating.” If you want to stress “right now,” you can add an adverb like gerade:
Ich esse gerade in der Kantine eine Suppe mit Tomaten und Gurken.

Could I omit Heute? If so, what changes in meaning or word order?

Yes, you can say Ich esse in der Kantine eine Suppe mit Tomaten und Gurken. You lose the explicit “today” reference. Word-order-wise, if you don’t start with Heute, you typically begin with Ich (subject), keeping the verb in second position.