Der Mobilfunktarif ist günstig.

Der is the definite article, meaning the.

In this sentence, der also tells you two grammar facts about Mobilfunktarif:

• it is masculine in grammatical gender
• it is in the nominative case, because it is the subject of the sentence

So:

• der Mobilfunktarif = the mobile phone plan / the mobile tariff

German very often combines nouns into a single compound word.

Here:

• Mobilfunk = mobile communications / cellular service
• Tarif = tariff, rate, plan

Together:

• Mobilfunktarif = mobile phone plan / mobile tariff

In English, these ideas are often written as separate words. In German, they are commonly written as one word.

It is in the nominative case.

That is because Mobilfunktarif is the subject of the sentence — the thing being described.

A simple way to see it:

• Der Mobilfunktarif = the subject
• ist = is
• günstig = inexpensive / good value

So the sentence structure is basically:

[subject] + [verb] + [description]

This is the normal word order for a simple German statement:

• Der Mobilfunktarif = subject
• ist = verb
• günstig = complement/adjective

German main clauses usually put the finite verb in the second position. Here, the subject comes first, so the verb comes right after it.

This is very similar to English:

• The mobile plan is inexpensive.

Because günstig is a predicate adjective here — it comes after the verb sein.

After sein, werden, and bleiben, adjectives usually do not change in German.

So you say:

• Der Tarif ist günstig.
• Die Tarife sind günstig.
• Das Angebot ist günstig.

Notice that günstig stays the same.

By contrast, when an adjective comes before a noun, it usually takes an ending:

• ein günstiger Tarif
• der günstige Tarif

Günstig usually means:

• inexpensive
• good value
• reasonably priced
• sometimes favorable, depending on context

In this sentence, the natural meaning is that the mobile phone plan is cheap in a positive sense or good value.

That is slightly different from English cheap, which can sometimes sound negative.

Both can mean cheap, but there is often a nuance:

• günstig = inexpensive, good value, favorably priced
• billig = cheap, but sometimes with a negative sense like low-quality or inferior

So in many situations, günstig sounds safer and more positive.

For example:

• Der Tarif ist günstig. = The plan is reasonably priced / good value.
• Der Tarif ist billig. = The plan is cheap.
  This may sound more neutral in some contexts, but it can also suggest lower quality.

The tricky part is the ü.

A rough guide:

• günstig sounds approximately like GUEN-stikh, but that is only an approximation

A few tips:

• ü is not the same as English u
• to make ü, try saying ee with your tongue, but round your lips as if saying oo
• the final -ig is often pronounced something like -ich in standard German, especially in careful speech

So many speakers pronounce günstig roughly like:

• GUEN-stich

But pronunciation varies somewhat by region.

Ist is the third-person singular form of the verb sein = to be.

Because der Mobilfunktarif is singular, German uses ist:

• der Tarif ist = the plan is

Compare:

• der Tarif ist günstig = the plan is inexpensive
• die Tarife sind günstig = the plans are inexpensive

So ist matches the singular subject.

Because all nouns are capitalized in German.

So in this sentence:

• Mobilfunktarif is capitalized because it is a noun
• günstig is not capitalized because it is an adjective

This is a basic spelling rule in German and is very important for reading and writing.

Yes, very often you can.

• Handytarif is very common in everyday German
• Mobilfunktarif can sound a bit more formal or technical

Both refer to a mobile phone plan, though exact usage can depend on context.

So these are both possible:

• Der Mobilfunktarif ist günstig.
• Der Handytarif ist günstig.

The first may sound a little more like advertising, business, or telecom language.