Physik ist interessant.

German often uses school subjects, academic fields, and languages without an article when speaking generally.

So:

• Physik ist interessant. = Physics is interesting.
• Mathematik ist schwer. = Mathematics is difficult.

You can sometimes use an article in other contexts, but in a general statement like this, no article is the normal choice.

Because Physik is grammatically singular.

The verb sein changes depending on the subject:

• ich bin
• du bist
• er/sie/es ist
• wir sind
• ihr seid
• sie/Sie sind

Since Physik is a singular noun, it takes ist.

German main clauses usually follow the verb-second rule, often called V2. That means the conjugated verb normally appears in the second position.

In this sentence:

• Physik = first element
• ist = second element
• interessant = rest of the sentence

So Physik ist interessant follows normal German word order.

Because interessant is being used as a predicate adjective, not an adjective directly before a noun.

Compare:

• Physik ist interessant.
  Here interessant comes after ist, so it gets no ending.

• eine interessante Vorlesung
  Here the adjective comes before a noun (Vorlesung), so it needs an ending: interessante.

This is a very important pattern in German: adjectives after sein, werden, bleiben, etc. do not take adjective endings.

Yes, Physik is a feminine noun: die Physik.

However, in this sentence, that does not affect the adjective interessant, because predicate adjectives do not change for gender.

So you say:

• Die Physik ist interessant.
• Die Geschichte ist interessant.
• Der Film ist interessant.
• Das Buch ist interessant.

The adjective stays interessant in all of these.

A learner-friendly pronunciation is roughly:

• Phy- sounds like fü
• -sik sounds like zeek or zik with a short i

A common approximation is fü-ZEEK, though the exact vowel quality depends on accent.

A few pronunciation notes:

• Ph in German is usually pronounced like f
• y in Physik sounds like the German ü
• the stress is on the second syllable: phy-SIK

A helpful approximation is:

in-te-re-SANT

The stress is usually on the last syllable: -sant.

A few notes:

• the r may sound different depending on the speaker or region
• the final t is pronounced
• it is not pronounced exactly like English interesting

Yes, you can. Die Physik ist interessant is grammatically correct.

The difference is mostly one of style and context:

• Physik ist interessant.
  More general, broad statement: Physics is interesting.

• Die Physik ist interessant.
  Also possible, but can sound a bit more specific or marked depending on context.

For a simple general statement about the subject, learners will most often see Physik ist interessant.

It can mean either, depending on context.

Physik can refer to:

• physics as an academic field
• physics as a school subject

So Physik ist interessant could mean:

• Physics is interesting in general
• Physics class/the subject physics is interesting

Usually the surrounding context tells you which meaning is intended.

Yes. That means Physics is an interesting science or Physics is an interesting field of science.

Notice the difference:

• Physik ist interessant.
  interessant is a predicate adjective, so no ending.

• eine interessante Wissenschaft
  Here interessante comes before the noun Wissenschaft, so it needs an adjective ending.

Physik is in the nominative case because it is the subject of the sentence.

In Physik ist interessant:

• Physik = subject, nominative
• ist = verb
• interessant = predicate adjective

Yes. This sentence pattern is very common:

[Noun] + ist + [adjective]

For example:

• Physik ist schwierig. = Physics is difficult.
• Physik ist spannend. = Physics is exciting.
• Physik ist wichtig. = Physics is important.
• Physik ist kompliziert. = Physics is complicated.

Again, the adjective stays unchanged after ist.