Åpningstiden er lang.

Because Norwegian very often combines nouns into one compound word.

åpningstiden = åpningstid + -en

And åpningstid itself is made of:

• åpning = opening
• tid = time

So Norwegian prefers one combined word where English often uses two or more words, such as opening time or opening hours.

The ending -en makes the noun definite, so it means the.

So:

• åpningstid = opening time / opening hours
• åpningstiden = the opening time / the opening hours

In Norwegian, the is often attached to the end of the noun instead of being a separate word.

Because Norwegian usually marks definiteness with an ending on the noun.

So instead of a separate article like English the, Norwegian often uses a suffix:

• tid = time
• tiden = the time

That is why åpningstiden already includes the idea of the.

Grammatically, it is singular definite.

That can feel a little strange to an English speaker, because English often says opening hours, which is plural. Norwegian can sometimes use a singular form where English prefers a plural expression.

So even if the English meaning shown to the learner is something like the opening hours, the Norwegian word here is still grammatically singular.

If you want an explicitly plural form, that would be:

• åpningstidene = the opening hours

Yes, in Bokmål you often can.

The noun tid is feminine, and many feminine nouns in Bokmål have two possible definite singular forms:

• åpningstiden
• åpningstida

Both are acceptable in many contexts. -en is often a bit more neutral or formal-looking in writing, while -a can sound more everyday or dialect-friendly, but both are common.

Because adjectives in Norwegian agree with the noun they describe.

Here, åpningstiden is a singular non-neuter noun, so the adjective takes the basic form:

• lang = used with singular common-gender nouns

Compare:

• åpningstiden er lang = singular common gender
• et møte er langt = singular neuter
• åpningstidene er lange = plural

So lang is the correct form here.

Because lang is an adjective, while lenge is usually an adverb.

In this sentence, the word is describing the noun åpningstiden, so you need an adjective:

• Åpningstiden er lang.

But lenge is used more with verbs:

• Butikken er åpen lenge. = The shop is open for a long time.

So:

• lang describes a noun
• lenge describes how long an action or state lasts

That is a very common distinction for learners.

It can correspond to either is or are in English, depending on the subject and the translation.

Norwegian verbs do not change form for singular and plural the way English verbs do. So er is used with:

• jeg er = I am
• han er = he is
• de er = they are

So er is just the present tense of å være (to be), regardless of number.

It is grammatical, but it may not always be the most natural choice, depending on what you mean.

Åpningstiden er lang sounds like you are talking about the length of one opening period, or treating the opening time as one whole stretch.

If you mean that a shop has long opening hours in general, Norwegian often more naturally says:

• Butikken har lange åpningstider.
• De har lange åpningstider.

And if you want the definite plural:

• Åpningstidene er lange.

So the given sentence is fine, but in everyday use many speakers would often choose a plural phrasing or a sentence with har.

A rough English-friendly approximation is:

AWP-nings-tee-den ehr lahng

A few notes:

• Å sounds roughly like aw in saw
• ng in åpning is like the ng in sing
• er is often pronounced somewhat like air or ehr, depending on dialect
• lang has a long vowel and ends with ng

The exact pronunciation varies by dialect, but that approximation is a useful starting point.