Nähgarn und Stopfwolle sind rot.

Because the subject is made of two things joined by und:

• Nähgarn
• Stopfwolle

Even though each noun is singular by itself, together they form a plural subject, so German uses the plural verb sind.

Compare:

• Das Nähgarn ist rot. = The sewing thread is red.
• Die Stopfwolle ist rot. = The darning wool is red.
• Nähgarn und Stopfwolle sind rot. = Sewing thread and darning wool are red.

Because rot comes after the verb sein and is being used as a predicate adjective.

In German, adjectives used this way do not take endings:

• Der Faden ist rot.
• Die Wolle ist rot.
• Nähgarn und Stopfwolle sind rot.

You only add adjective endings when the adjective comes before a noun:

• rotes Nähgarn
• rote Stopfwolle

So:

• Das Nähgarn ist rot. ✅
• Das ist rotes Nähgarn. ✅

German often leaves out articles when talking about things in a general way, especially with materials, substances, or category-like nouns.

So Nähgarn und Stopfwolle sind rot can mean something like:

• sewing thread and darning wool are red
• the sewing thread and darning wool are red

The exact sense depends on context.

If you want to refer to specific items, you can include articles:

• Das Nähgarn und die Stopfwolle sind rot.

That sounds more clearly like the sewing thread and the darning wool.

They are in the nominative case, because they are the subject of the sentence.

You can think of the structure like this:

• Nähgarn und Stopfwolle = subject
• sind = verb
• rot = predicate adjective

So this is a very standard subject + verb + complement sentence.

Both nouns are neuter:

• das Nähgarn
• das Stopfwolle is not standard; the usual dictionary form is die Stopfwolle? Actually, Stopfwolle is normally die Stopfwolle.

So the genders are:

• das Nähgarn = neuter
• die Stopfwolle = feminine

That said, in this sentence the gender does not affect the form of rot, because rot is a predicate adjective here and stays unchanged.

It also does not affect the verb, because the two nouns together make a plural subject, so the verb is sind.

Because they are nouns, and in German all nouns are capitalized.

This is one of the most noticeable spelling differences from English.

Examples:

• das Haus
• die Wolle
• der Tisch
• Nähgarn
• Stopfwolle

But adjectives and verbs are usually not capitalized unless they begin the sentence:

• rot
• sind

Yes. German very often builds longer nouns by combining shorter words.

• Nähgarn = Näh-

  • Garn
  • nähen = to sew
  • Garn = yarn/thread
  • so Nähgarn is sewing thread

• Stopfwolle = Stopf-

  • Wolle
  • stopfen = to darn/mend
  • Wolle = wool
  • so Stopfwolle is darning wool

This is very common in German vocabulary, and learning to spot the parts of a compound word is very helpful.

The main thing learners usually notice is the ä.

A simple approximation is:

• Näh- sounds somewhat like neh in English, but with a pure vowel
• Garn sounds roughly like garn, but with a German r

So Nähgarn is approximately NEH-garn.

Also note:

• äh is long here
• German r may sound different depending on region

If you already know nähen (to sew), the first part is the same root.

Yes. That is perfectly grammatical.

The difference is mainly one of specificity:

• Nähgarn und Stopfwolle sind rot.
  More general, label-like, or context-dependent.

• Das Nähgarn und die Stopfwolle sind rot.
  More clearly refers to specific thread and specific darning wool.

Both are possible, depending on what you mean.

Neither. In this sentence, rot does not show singular/plural marking at all.

That is because it is a predicate adjective after sein. Predicate adjectives in German stay in the basic form:

• Der Apfel ist rot.
• Die Äpfel sind rot.
• Das Kleid ist rot.
• Die Kleider sind rot.

So rot stays rot no matter what the subject is.

Yes. German allows some flexibility, as long as the finite verb stays in the second position in a main clause.

For example, you could say:

• Rot sind Nähgarn und Stopfwolle.

That is grammatical, but it sounds more marked or literary, because rot is being emphasized.

The most neutral version is:

• Nähgarn und Stopfwolle sind rot.

Yes: by joining the two nouns, und creates a compound subject, and that is why the verb becomes plural:

• Nähgarn ist rot.
• Stopfwolle ist rot.
• Nähgarn und Stopfwolle sind rot.

So und is not just adding another noun; it also changes the agreement of the verb.

Grammatically, Nähgarn and Stopfwolle are each singular nouns, but they are being used more like materials/categories than countable individual objects.

So the sentence does not necessarily mean just one single piece of thread and one single piece of wool. It can describe them as types of material or items in context.

That is why English may translate this in different natural ways depending on context:

• sewing thread and darning wool are red
• the sewing thread and darning wool are red

German often uses singular mass nouns this way.