Für die Tomaten brauche ich den Dosenöffner, weil ich frische Tomaten vergessen habe.

Because für always takes the accusative case, and Tomaten is plural, so the article is die.

• nominative plural: die Tomaten
• accusative plural: die Tomaten

In the plural, nominative and accusative look the same here, so the main clue is that für requires accusative.

Because Dosenöffner is the direct object of brauchen.

The verb brauchen means to need, and what you need is put in the accusative:

• der Dosenöffner = nominative
• den Dosenöffner = accusative

So:

• Ich brauche den Dosenöffner. = I need the can opener.

It is a compound noun:

• Dose = can / tin
• Öffner = opener

So Dosenöffner literally means can-opener.

A small detail: in compounds, the first part often changes form slightly. Here Dose becomes Dosen- before Öffner.

German often moves a phrase to the beginning of the sentence for emphasis or topic.

So:

• Für die Tomaten brauche ich den Dosenöffner.
• Ich brauche den Dosenöffner für die Tomaten.

Both are grammatical, but the first version puts extra focus on for the tomatoes.

Because German main clauses follow the verb-second rule, once Für die Tomaten is placed first, the conjugated verb must come next:

• Für die Tomaten | brauche | ich ...

That is why it is brauche ich, not ich brauche.

Because German main clauses usually put the finite verb in second position.

In this sentence, the first position is already taken by Für die Tomaten, so the verb comes next:

• 1st position: Für die Tomaten
• 2nd position: brauche
• then: ich

This is a very common German word order pattern.

Because weil introduces a subordinate clause, and in standard German, subordinate clauses send the conjugated verb to the end.

The verb form here is the perfect tense:

• past participle: vergessen
• auxiliary: habe

In a weil clause, both verbal parts go to the end, with the participle before the auxiliary:

• weil ich frische Tomaten vergessen habe

That is standard written and careful spoken German.

Because German often leaves out the article when talking about an indefinite plural amount of something.

Compare:

• Ich habe Tomaten gekauft. = I bought tomatoes.
• Ich habe frische Tomaten vergessen. = I forgot fresh tomatoes.

This is similar to English, where you can also say I forgot fresh tomatoes without saying some fresh tomatoes.

Because the adjective is modifying a plural noun with no article in the accusative.

When there is no article, the adjective has to carry more grammatical information itself. So here we get:

• frische Tomaten

This is called a strong adjective ending. For accusative plural with no article, the ending is typically -e.

It is the perfect tense (Perfekt), which is very common in spoken German for talking about the past.

It is formed with:

• haben as the auxiliary
• the past participle vergessen

So:

• ich habe vergessen = I forgot / I have forgotten

In everyday spoken German, this is often preferred over the simple past:

• ich vergaß sounds much more literary or formal.

In this sentence, vergessen most likely means failed to remember to bring or buy fresh tomatoes.

So the idea is probably:

• I need the can opener for the tomatoes because I forgot fresh tomatoes.

In natural context, that usually means the speaker forgot to get fresh tomatoes, so they are using canned tomatoes instead.

German vergessen can cover several English ideas:

• forget a fact
• forget an object
• forget to do something

The exact meaning depends on context.

Not necessarily. In fact, the sentence strongly suggests a contrast.

A natural reading is:

• für die Tomaten = for the tomatoes being used now, probably canned tomatoes
• frische Tomaten = fresh tomatoes that the speaker intended to have but forgot

So the sentence implies something like: the speaker needs a can opener because they do not have fresh tomatoes and must use canned ones instead.

Here für means something like for or for dealing with / for preparing.

So:

• Für die Tomaten brauche ich den Dosenöffner.

means the can opener is needed for the tomatoes.

It does not mean possession. It expresses purpose or use.

Yes, that is possible, but it changes the feel slightly.

• frische Tomaten = fresh tomatoes, indefinite
• die frischen Tomaten = the fresh tomatoes, more specific

Using die would suggest that both speaker and listener know exactly which fresh tomatoes are meant.

Without the article, the sentence sounds more general and natural in many everyday situations.

In colloquial spoken German, you may sometimes hear things like:

• ..., weil ich habe die Tomaten vergessen.

But in standard German, especially in writing and careful speech, the correct form is:

• ..., weil ich die Tomaten vergessen habe.

So for learners, the safest rule is:

• after weil, put the verb at the end

Yes. That is also grammatical.

German allows some flexibility in word order, especially with adverbial phrases like für die Tomaten. The difference is mostly about focus:

• Für die Tomaten brauche ich den Dosenöffner.
  Focuses more on for the tomatoes
• Ich brauche den Dosenöffner für die Tomaten.
  More neutral
• Ich brauche für die Tomaten den Dosenöffner.
  Also possible, with a slightly different rhythm

All three can work depending on context.

Yes, Tomaten is the same word both times, and it is pronounced the same way. The difference is only in the surrounding grammar and meaning in context.

It is the plural form of die Tomate.

There is a comma before weil, and it is required.

In German, subordinate clauses introduced by words like weil, dass, wenn, and obwohl are normally separated from the main clause by a comma.

So this punctuation is standard:

• ..., weil ich frische Tomaten vergessen habe.