Die Erdbeeren sind manchmal sauer, die Melone schmeckt dagegen fast immer süß.

Because die can mean different things in German:

• die Erdbeeren = the strawberries
  Here, die is the plural definite article.
• die Melone = the melon
  Here, die is the feminine singular definite article.

So even though the form is the same, the grammar is different:

• die Erdbeere = the strawberry
• die Erdbeeren = the strawberries
• die Melone = the melon

German articles often overlap like this, so you have to tell from the noun and verb whether it is singular or plural.

Because the subjects are different, and the sentence uses two different verbs:

• Die Erdbeeren sind manchmal sauer
  sind comes from sein = to be
• die Melone schmeckt dagegen fast immer süß
  schmeckt comes from schmecken = to taste

Also:

• Erdbeeren is plural, so it takes sind
• Melone is singular, so it takes schmeckt

So the sentence is contrasting:

• strawberries are sometimes sour
• the melon tastes almost always sweet

Yes, it could. You could say:

• Die Erdbeeren schmecken manchmal sauer.

That would mean The strawberries sometimes taste sour.

The version with sind focuses more on the strawberries’ quality or state:

• They are sometimes sour

The version with schmecken focuses more directly on flavor:

• They sometimes taste sour

In many contexts, both are natural. German often uses both sein + adjective and schmecken + adjective, depending on what the speaker wants to emphasize.

Here, dagegen means something like:

• in contrast
• on the other hand
• by contrast

So:

• Die Erdbeeren sind manchmal sauer, die Melone schmeckt dagegen fast immer süß.

means that the second clause is being contrasted with the first one.

A very natural English sense is:

• Strawberries are sometimes sour; the melon, by contrast, tastes almost always sweet.

German allows adverbs like dagegen to move around, but their position affects style and emphasis.

In this sentence:

• die Melone schmeckt dagegen fast immer süß

the normal order is:

• subject: die Melone
• verb: schmeckt
• adverb: dagegen
• time expression: fast immer
• adjective/complement: süß

You could also say:

• Die Erdbeeren sind manchmal sauer, dagegen schmeckt die Melone fast immer süß.

That puts stronger emphasis on the contrast. But the original version sounds very natural and smooth.

fast immer means almost always.

• fast = almost
• immer = always

So:

• fast immer süß = almost always sweet

It suggests that the melon is sweet in nearly every case, but not absolutely 100% of the time.

Because they are predicate adjectives, not adjectives directly in front of a noun.

Compare:

• Die Erdbeeren sind sauer.
  Here, sauer comes after sein, so it does not take an ending.
• die sauren Erdbeeren
  Here, the adjective comes before the noun, so it does take an ending.

The same applies to:

• Die Melone schmeckt süß.
• die süße Melone

So in your sentence, sauer and süß stay in their basic form because they are linked to the noun by sein or schmecken.

Because the subject is die Melone, which is third person singular.

The verb schmecken is conjugated like this in the present tense:

• ich schmecke
• du schmeckst
• er/sie/es schmeckt
• wir schmecken
• ihr schmeckt
• sie/Sie schmecken

Since die Melone = she/it grammatically, the correct form is schmeckt.

Because the sentence contains two main clauses joined together:

1. Die Erdbeeren sind manchmal sauer
2. die Melone schmeckt dagegen fast immer süß

In German, when two independent main clauses are connected without a conjunction like und, a comma is used.

So the comma helps mark the break between the two complete thoughts.

Usually, with a sentence like this, it can mean strawberries and melons in a general sense, even though German uses the definite article.

German often uses the definite article where English might use no article:

• Die Erdbeeren sind manchmal sauer.
• literally: The strawberries are sometimes sour
• natural meaning: Strawberries are sometimes sour

Likewise:

• die Melone can mean melon in a general, typical sense, depending on context.

So the article does not always mean a very specific, previously mentioned item the way English the often does.

A helpful way to see it is:

• die Melone = subject
• schmeckt = verb
• dagegen = contrast adverb
• fast immer = time/frequency expression
• süß = complement describing the taste

So the structure is roughly:

Subject + Verb + Adverb + Time expression + Adjective

German main clauses usually keep the finite verb in second position, and that is exactly what happens here:

• die Melone | schmeckt | dagegen | fast immer | süß

süß means sweet.

The letter ß is called Eszett or scharfes S. In modern German spelling, it usually represents a strong s sound after a long vowel or diphthong.

So:

• süß is pronounced roughly like zyooss or syooss, depending on accent approximation
• it is not pronounced like English z exactly, but more like an s sound

Also, ß is not the same as b. English learners sometimes misread it at first, but it is a special German letter.

Yes. manchmal means sometimes, and it is an adverb of frequency.

In:

• Die Erdbeeren sind manchmal sauer

it tells you how often the strawberries are sour.

German often places frequency adverbs like manchmal, oft, immer, nie in the middle part of the sentence, often after the verb in a simple main clause.

So this is a very normal placement:

• Die Erdbeeren sind manchmal sauer.

You could also move it for emphasis in some contexts, but the given word order is standard and natural.