Glasaffaldet er tungt.

Because Danish, like English, often makes compound nouns by joining nouns together. Here:

• glas = glass
• affald = waste

So glasaffald means glass waste as a single compound noun, not two separate words.

Then Danish adds the definite ending directly to the noun:

• glasaffald = glass waste
• glasaffaldet = the glass waste

So the whole word is built like this:

• glas + affald + et

-et is the definite article ending for a singular neuter noun in Danish.

Unlike English, Danish usually puts the at the end of the noun instead of using a separate word.

Compare:

• et affald = a waste item / waste
• affaldet = the waste
• glasaffaldet = the glass waste

So -et here works like English the.

Because affald is a neuter noun in Danish, and neuter nouns take et in the singular.

Danish nouns have two grammatical genders:

• en-words (common gender)
• et-words (neuter)

Since the head noun is affald, the compound follows that noun’s gender:

• et affald
• glasaffaldet

Even though glas is the first part of the compound, the gender is decided by the last/main noun, which is affald.

Because adjectives in Danish often agree with the noun they describe.

Here, glasaffaldet is:

• singular
• neuter
• used with er in a predicate structure

So the adjective takes the neuter singular form:

• tung = heavy
• tungt = heavy, when describing a singular neuter noun

Examples:

• Bogen er tung. = The book is heavy.
  (bog is a common-gender noun)
• Glasaffaldet er tungt. = The glass waste is heavy.
  (glasaffald is neuter)

No. The meaning is the same. The difference is grammatical, not semantic.

• tung = basic form
• tungt = form used with a singular neuter noun

So in this sentence, tungt does not mean a special kind of heavy. It is just the form required by the grammar.

Er is the present tense of at være, which means to be.

So:

• er = is / are / am, depending on the subject in English

In this sentence, it links the subject glasaffaldet with the adjective tungt:

• Glasaffaldet = subject
• er = is
• tungt = heavy

This is a very common Danish sentence pattern:

• X er Y = X is Y

The dictionary form is usually glasaffald.

Dictionaries normally list nouns in the indefinite singular form, not the definite form. So you would usually look up:

• glasaffald rather than glasaffaldet
• affald rather than affaldet

Likewise, for the adjective, the dictionary form is usually:

• tung rather than tungt

No. This is the normal Danish word order for a simple statement:

• Subject + verb + complement

So:

• Glasaffaldet = subject
• er = verb
• tungt = adjective complement

This is very similar to English:

• The glass waste is heavy.

Danish word order can change in other sentence types, but this sentence is straightforward.

Because Danish does not capitalize all nouns.

Unlike German, Danish only capitalizes:

• the first word of a sentence
• proper nouns

So Glasaffaldet has a capital G only because it begins the sentence. If it appeared in the middle of a sentence, it would normally be written glasaffaldet.

Yes. Affald is often used like a mass noun, similar to English waste or trash.

So Danish can use a singular form even when talking about a quantity of material:

• glasaffaldet er tungt = the glass waste is heavy

This does not have to mean one single object. It can refer to the material or collection as a whole.

No, not in normal Danish.

If you mean glass waste, it should be written as the compound glasaffald. With the definite ending, it becomes glasaffaldet.

Writing glas affaldet would look wrong because Danish normally keeps this kind of compound as one word.