Avlingen er stor.

Because Norwegian usually puts the definite article at the end of the noun.

• avling = indefinite singular, like a crop / a yield
• avlingen = definite singular, like the crop / the yield

So -en here is the definite ending.

In Bokmål, this noun is treated here as an en-word:

• en avling
• avlingen

Because adjectives used after er must agree with the noun in gender and number.

Here, avlingen is:

• singular
• an en-word noun

So the adjective form is stor.

Compare:

• avlingen er stor = singular en-word
• huset er stort = singular neuter
• avlingene er store = plural

This is a very common question.

In predicative position — that is, when the adjective comes after a verb like er — the adjective agrees with gender and number, but not with definiteness.

So you say:

• Avlingen er stor

Not:

• Avlingen er store

But when the adjective comes before the noun, definiteness does matter:

• den store avlingen = the big crop / the large yield

So:

• after er → stor
• before the noun in a definite phrase → store

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

So this is a very basic Norwegian sentence pattern:

• subject + er + adjective

Examples:

• Avlingen er stor
• Boken er interessant
• Huset er gammelt

With a word like avling, stor usually means large in amount or abundant, not necessarily physically large in shape.

So the idea is closer to:

• a large yield
• a big crop
• an abundant harvest

If you wanted to talk about quality instead, Norwegian might use words like:

• god = good
• bra = good / fine

So avlingen er stor focuses on size/amount, not on whether it is good quality.

They are related, but not the same.

• avling = the crop, yield, or produce
• høst = harvest, autumn, or the act/season of harvesting, depending on context

So avlingen is the thing produced, while høst is more about the harvest itself or the season.

Very roughly:

• avlingen er stor = the yield/crop is large
• høsten begynner snart = the harvest starts soon

You would change both the noun and the adjective.

Examples:

• Avlingene er store = the crops / yields are large
• Avlinger er store = crops / yields are large

Notice that in the plural, the adjective becomes store.

So the pattern is:

• singular en-word → stor
• singular neuter → stort
• plural → store

Yes, but in a normal statement this order is the most basic:

• Avlingen er stor

If you add something at the beginning, Norwegian still keeps the verb in second position:

• I år er avlingen stor = This year, the crop is large

Not:

• I år avlingen er stor

That verb-second rule is very important in Norwegian main clauses.