Wenn die Banane zu weich ist, esse ich lieber eine Birne.

Here, wenn introduces a condition, so the most natural English meaning is if.

• Wenn die Banane zu weich ist ... = If the banana is too soft ...

German wenn can also mean when in other contexts, especially for repeated situations, but in this sentence a native English speaker would usually translate it as if.

Because wenn starts a subordinate clause. In German, subordinate clauses usually send the conjugated verb to the end.

So:

• die Banane ist zu weich = main clause
• wenn die Banane zu weich ist = subordinate clause, so ist moves to the end

This is one of the most important German word-order patterns to learn.

In German main clauses, the conjugated verb must be in second position.

When the sentence begins with the wenn-clause, that whole clause counts as position 1. So the verb in the main clause must come next:

• Wenn die Banane zu weich ist, esse ich lieber eine Birne.

Structure:

1. Wenn die Banane zu weich ist
2. esse
3. ich
4. the rest of the sentence

So it is not that the verb is randomly moving; it is following the normal verb-second rule.

Yes. In German, a subordinate clause introduced by wenn is normally separated from the main clause with a comma.

So this is correct:

• Wenn die Banane zu weich ist, esse ich lieber eine Birne.

The comma is not optional here.

Here zu means too.

• zu weich = too soft
• sehr weich = very soft

That is an important difference:

• sehr weich just describes something as very soft
• zu weich means it is more soft than desired or excessively soft

Also, this zu is not the same as the zu used before an infinitive, as in zu essen.

Because weich is a predicate adjective here. It comes after ist, not directly before a noun.

In German, adjectives used after verbs like sein, werden, and bleiben do not take adjective endings.

• Die Banane ist zu weich.
• eine zu weiche Banane

Compare those:

• After ist: weich
• Before the noun Banane: weiche

Lieber means rather or preferably, and it expresses preference.

So:

• Ich esse lieber eine Birne. = I’d rather eat a pear / I prefer to eat a pear

It is related to gern:

• Ich esse gern Birnen. = I like eating pears.
• Ich esse lieber Birnen als Bananen. = I prefer pears to bananas.

A useful way to think about it:

• gern = like doing something
• lieber = prefer doing something

This is about definite versus indefinite articles.

• die Banane = the banana
• eine Birne = a pear

So the speaker is talking about a particular banana, but then mentions a pear as an alternative.

Also, both Banane and Birne are feminine nouns, which is why their dictionary form is:

• die Banane
• die Birne

Because Birne is feminine, and the direct object here is in the accusative case.

The verb essen takes a direct object, so eine Birne is accusative. For feminine nouns, the indefinite article stays eine in both nominative and accusative.

Compare:

• eine Birne = nominative feminine
• eine Birne = accusative feminine

But masculine changes:

• ein Apfel = nominative masculine
• einen Apfel = accusative masculine

So:

• Ich esse eine Birne.
• Ich esse einen Apfel.

Yes, absolutely. That is also correct German.

• Wenn die Banane zu weich ist, esse ich lieber eine Birne.
• Ich esse lieber eine Birne, wenn die Banane zu weich ist.

Both mean the same thing overall. The difference is mainly in emphasis and sentence flow.

Word order changes because the main clause is now first:

• Ich esse ... = normal main-clause order
• then the wenn-clause comes after it

You still need the comma before the subordinate clause.