Núna er góður tími fyrir göngutúr.

Icelandic has no separate indefinite article like English a/an.

• góður tími can mean good time or a good time, depending on context.
• If you want the good time, you use a definite form, usually by adding a suffix: tími → tíminn (the time).

So góður tími naturally corresponds to English a good time here.

Adjectives in Icelandic must agree with the noun in:

• Gender (masculine, feminine, neuter)
• Number (singular/plural)
• Case (nominative, accusative, dative, genitive)

Here, tími is:

• masculine
• singular
• nominative (it’s the subject)

So góður is the masculine singular nominative form of the adjective góður (good).

Compare:

• góður tími – good time (m. nom. sg.)
• góð stelpa – good girl (f. nom. sg.)
• gott veður – good weather (n. nom. sg.)
• Ég á góðan tíma – I have a good time (here góðan is masculine accusative).

In Núna er góður tími fyrir göngutúr, tími is in the nominative singular, because it is the subject of the sentence.

The basic declension of tími is:

• Nominative: tími – subject (e.g. Tími er peningar – Time is money)
• Accusative: tíma – direct object (e.g. Ég á ekki tíma – I don’t have time)
• Dative: tíma – often after certain prepositions (e.g. um tíma – for a while)
• Genitive: tíma – possession/“of time” (e.g. mikið af tíma – a lot of time)

In your sentence, it stays as tími because subjects are nominative.

The subject is góður tími, not Núna.

• Núna is an adverb (“now”), giving information about time.
• Icelandic main clauses are typically verb-second (V2):
  
  • Some element (here, the adverb Núna) comes first,
  • then the finite verb (er),
  • then the subject (góður tími).

So the underlying structure is like [Now] [is] [a good time] [for a walk]. The subject is the noun phrase góður tími.

In this sentence, fyrir means for, in the sense of “a good time for X” (suitable time for something).

• With this meaning (benefit/purpose), fyrir takes the accusative case.
• göngutúr is therefore in the accusative singular after fyrir.

For this particular noun, the nominative and accusative forms are the same (göngutúr), so you don’t see the change in form, but grammatically it’s accusative.

Note that fyrir can take accusative or dative with different meanings (e.g. fyrir húsinu – in front of the house, dative; fyrir húsið – in front of the house as a direction, accusative), but here it’s the “for (some purpose)” use.

göngutúr means “a walk” or “a stroll”, usually in the sense of going out for a walk.

It’s a compound noun:

• göngu-: from ganga (to walk) / ganga (a walk, a walking), here in a stem related to the genitive göngu
• túr: a trip, tour, or outing

Literally, it’s like “walking-trip”, which fits the idea of going out for a walk.
You’ll often see expressions like:

• að fara í göngutúr – to go for a walk
• Ég fer oft í göngutúr. – I often go for a walk.

Yes, Núna er góður tími að fara í göngutúr is natural and common. It means:

• “Now is a good time to go for a walk.”

Your original sentence:

• Núna er góður tími fyrir göngutúr.
  – “Now is a good time for a walk.”

The difference is:

• fyrir göngutúr uses a noun phrase (“for a walk” as an activity in general).
• að fara í göngutúr uses a verb phrase (“to go for a walk” as an action).

In practice, the meaning is almost the same, and both are very idiomatic.

You can say either:

• Núna er góður tími fyrir göngutúr.
• Nú er góður tími fyrir göngutúr.

Both are correct and would be understood as “Now is a good time for a walk.”

Subtle differences:

• núna: often a bit more concrete and immediate, “right now, at this point in time”.
• nú: can also have a discourse or emotional flavor (“now, listen”, “now then”), and is sometimes felt as slightly more formal or rhetorical, depending on context.

In this sentence, the difference is small; many speakers would use both interchangeably.

To make “the good time” explicit, you make both the adjective and the noun definite:

• Núna er góði tíminn fyrir göngutúr.
  – Literally: “Now is the good time for a walk.”

Grammar points:

• tími (time) → tíminn (the time): definite -inn suffix on the noun.
• góður (good, strong form) → góði (good, weak definite form), agreeing with a definite masculine singular noun.

This sounds like you are contrasting now with other times: Now is the (particular) good time for a walk (not earlier or later).

The normal way to negate this sentence is:

• Núna er ekki góður tími fyrir göngutúr.
  – “Now is not a good time for a walk.”

Word order:

• Núna – adverb in first position
• er – verb (must be in second position in main clauses)
• ekki – the negative word, placed after the verb
• góður tími fyrir göngutúr – the rest of the predicate

So the pattern is: [Adverb] + [Verb] + ekki + [rest].

er is the present indicative form of vera (to be). It’s used for plain factual statements:

• Núna er góður tími fyrir göngutúr.
  – Now is a good time for a walk.

sé is the present subjunctive form and appears mainly in:

• subordinate clauses after verbs like to think, believe, hope, want:
  
  • Ég held að núna sé góður tími fyrir göngutúr.
    – I think that now is a good time for a walk.
• hypothetical or less certain statements.

So you use er in a direct main-clause statement, and sé inside certain dependent or more tentative clauses.

Change only the verb vera:

• Past (var):

  • Áðan var góður tími fyrir göngutúr.
    – Earlier was a good time for a walk.
  • Í gær var góður tími fyrir göngutúr.
    – Yesterday was a good time for a walk.

• Future (verður):

  • Seinna verður góður tími fyrir göngutúr.
    – Later will be a good time for a walk.
  • Á morgun verður góður tími fyrir göngutúr.
    – Tomorrow will be a good time for a walk.

The rest (góður tími fyrir göngutúr) stays the same; only er → var / verður changes.

Very roughly for an English speaker (stressed syllables in CAPS):

• Núna – NÚ-na

  • ú like long oo in food; stress on NÚ.

• er – like “air” but shorter, with a rolled/trilled r.

• góður – GOH-thur (approx.)

  • ó like long o in go (held a bit longer).
  • ð like the th in this (voiced).
  • final -ur is short and the r is rolled.

• tími – TEE-mi

  • í like long ee in see.
  • Stress on TEE.

• fyrir – approx. FIR-ir

  • Both y and i are like a short i (as in bit, but more front).
  • The r is rolled/flapped.

• göngutúr – approx. GÖNG-uh-toor

  • göng-: g is hard (like g in go), ö like German ö in schön or like saying e in bed with rounded lips, ng as in sing.
  • -u- in the middle is a short, central u sound.
  • túr: ú like long oo in food; r rolled.
  • Stress on the first part: GÖNG-u-túr.

All content words in Icelandic are stressed on the first syllable, which helps with rhythm: NÚ-na er GÓ-ður TÍ-mi FYR-ir GÖNG-u-túr.