Learn Hardcore Icelandic: Stiginn er sleipur í dag.

So Stiginn er sleipur í dag is literally The staircase is slippery today, but in everyday English we’d more naturally say The stairs are slippery today. The number difference is just a quirk of how each language conceptualizes it.

What is the base form and gender of stiginn?

The base (dictionary) form is:

stigi – masculine noun, meaning stair, staircase, ladder.

A simplified singular declension:

Nominative: stigi – a staircase (subject form)
Accusative: stiga – staircase as a direct object
Dative: stiga
Genitive: stiga

Definite forms in the singular:

Nominative: stiginn – the staircase (subject)
Accusative: stigann – the staircase (object)
Dative: stiganum
Genitive: stigans

How does Icelandic express the in stiginn?

Icelandic usually attaches the definite article as a suffix to the noun instead of having a separate word like English the.

For this noun:

stigi = a staircase
stiginn = the staircase

Here -inn is the masculine nominative singular definite ending. Other genders/cases use different endings, e.g.:

bíll (a car, masc. nom. sg.) → bíllinn (the car)
hús (a house, neut. nom. sg.) → húsið (the house)
bók (a book, fem. nom. sg.) → bókin (the book)

Why is stiginn in the nominative case?

Because stiginn is the subject of the sentence.

The verb er (is) is a linking verb (copula).
In Icelandic, the subject of the sentence is in the nominative case.
The describing word (sleipur) also appears in nominative to agree with it.

So:

Stiginn (nom. subject)
er (linking verb)
sleipur (predicative adjective in nominative, agreeing with stiginn)

What form of the verb is er, and what does it correspond to in English?

er is the 3rd person singular present tense of the verb vera (to be), corresponding to is.

A mini present-tense paradigm:

ég er – I am
þú ert – you are (singular)
hann / hún / það er – he / she / it is
við erum – we are
þið eruð – you are (plural)
þeir / þær / þau eru – they are

So Stiginn er sleipur í dag = The staircase is slippery today.

Why is the adjective sleipur used, and not sleip, sleipi, or sleipa?

Adjectives in Icelandic must agree with the noun in:

gender
number
case

and they appear in either strong or weak declension.

Here:

stiginn is masculine, singular, nominative.
After er, the adjective is used predicatively (after to be), and in that use it normally takes the strong form.

Strong nominative singular forms of sleipur:

Masculine: sleipur
Feminine: sleip
Neuter: sleipt

So we need sleipur to match a masculine nominative singular noun:

Stiginn er sleipur. – The staircase is slippery.

Forms like sleipi or sleipa are other case/gender/declension combinations and would be wrong here.

What exactly does sleipur mean, and are there common synonyms?

Literal meaning:

sleipur = slippery, slick (physically smooth so that you can easily slide on it).

Typical uses:

Gólfið er sleipt. – The floor is slippery.
Vélin er sleip af olíu. – The machine is slick with oil.

Common near-synonyms (for physical slipperiness):

háll – also slippery (very common for icy surfaces: Vegalengjan er mjög háll – The road is very slippery).
blautur – wet, which can imply slippery but doesn’t inherently mean slippery.

Figurative use:

sleipur can also mean slick, slippery (as a person), i.e. cunning or hard to pin down:
- Hann er sleipur pólitíkus. – He’s a slippery/slick politician.

In your sentence it’s clearly the physical meaning: the staircase is slippery to walk on.

How does í dag work grammatically, and why isn’t there a word for the?

í dag literally means in day, but idiomatically today.

í = preposition in.
dagur = day (masculine noun, base form).
In time expressions, í governs the accusative:
- dagur (nom.) → dag (acc.).

So:

í dag = in (the) day → today.

There is no definite article here; Icelandic just uses í dag as a fixed time expression meaning today, similar to English today rather than the day of today.

Can I change the word order, like Í dag er stiginn sleipur? Does it change the meaning?

Yes, that word order is correct, and the meaning is essentially the same.

Neutral order:

Stiginn er sleipur í dag. – The staircase is slippery today.
(Subject–verb–complement–time)

With time expression fronted:

Í dag er stiginn sleipur. – Today the staircase is slippery.

Icelandic is a verb-second (V2) language in main clauses:

If you move í dag to the front for emphasis (on today), the verb (er) must still be the second element, so it shifts in front of stiginn.

Nuance:

Stiginn er sleipur í dag. – neutral statement.
Í dag er stiginn sleipur. – slightly stronger emphasis on “today (as opposed to other days).”

How do you pronounce Stiginn er sleipur í dag?

An approximate IPA transcription:

Stiginn er sleipur í dag → [ˈstɪːjɪn ɛr ˈstleiːpʏr iː ˈtaːɣ]

Piece by piece:

Stiginn – [ˈstɪːjɪn]
- gi is like gy ([j]) in English yes.
er – [ɛr], like air but shorter.
sleipur – [ˈstleiːpʏr]
- ei roughly like English ay in day.
- u here is [ʏ], like German ü in müde.
í – [iː], like a long ee in see.
dag – [taːɣ]
- d is often pronounced more like [t].
- Final g is a voiced velar fricative [ɣ], similar to a softer, voiced version of the German ch in Bach.

Stress falls on the first syllable of each content word: STI-ginn, SLEI-pur, DAG.

If I want to talk about several staircases or outdoor steps, how would I say The stairs are slippery today?

You have a few options, depending on what you mean:

One staircase (most common reading of your original sentence):
- Stiginn er sleipur í dag. – The staircase is slippery today.
  (English often uses plural stairs for this.)
Several staircases:
- Stigarnir eru sleipir í dag. – The staircases are slippery today.
  - stigi → stigar (nom. pl.) → stigarnir (definite nom. pl.)
Outdoor steps / front steps (very common expression):
- Tröppurnar eru sleipar í dag. – The steps are slippery today.
  - trappa (a step) → tröppur (stairs/steps) → tröppurnar (the steps).

All of these can be followed by í dag to indicate today.

Is Stiginn er sleipur í dag more like is slippery or is being slippery in English?

Icelandic does not have a separate progressive form like English is being.

er
- adjective covers both a state and a temporary condition.
The adverbial í dag (today) shows clearly that this is about a temporary situation.

So Stiginn er sleipur í dag can correspond to:

The staircase is slippery today.
(specifically today, not as a permanent property)

You cannot say a literal equivalent of is being slippery in normal Icelandic; you just use the simple present as in the sentence you have.