Ein regla er sú að við borðum saman á kvöldin.

The numeral “one” changes its form depending on the gender of the noun:

• einn – masculine
• ein – feminine
• eitt – neuter

The noun regla (rule) is feminine, so ein has to agree with it:

• ein regla = one rule
• einn stóll = one chair (masc.)
• eitt borð = one table (neuter)

Regla means “rule” or “regulation.”

Its grammatical information:

• Gender: feminine
• Nominative singular: regla
• Definite form: reglan (the rule)

Because it is feminine, it takes ein (not einn, not eitt).

Sú is the feminine nominative singular form of the demonstrative pronoun sá / sú / það (roughly that).

In the structure:

Ein regla er sú að …

you can understand it as something like:

• “One rule is this: that we eat together in the evenings.”
• or more loosely, “One rule is the one that we eat together in the evenings.”

Grammatically:

• sú refers back to regla (which is feminine), so it must also be feminine.
• The að‑clause (að við borðum saman á kvöldin) then explains what that rule is.

This pattern “X er sú að …” is quite common and a bit more formal or explicit than just saying “X er að …”.

Yes, you can, and it is grammatically correct:

• Ein regla er að við borðum saman á kvöldin.

This sounds a bit more straightforward and slightly less formal.
With sú, the sentence feels a bit more like:

• “One rule is this, namely that we eat together in the evenings.”

Without sú, it is closer to:

• “One rule is that we eat together in the evenings.”

Both versions are natural; the one with sú is just a little more “spelled out.”

Here, að is not the “to” you know from infinitives like að borða (to eat).
In this sentence, að is a conjunction meaning “that”, introducing a subordinate clause:

• að við borðum saman á kvöldin = “that we eat together in the evenings”

So:

• að borða – to eat (infinitive marker)
• að við borðum – that we eat (conjunction introducing a clause)

Same spelling, different grammatical role.

The verb is að borða (to eat).
Borðum is:

• 1st person plural, present tense → “we eat”

Present tense forms of borða:

• ég borða – I eat
• þú borðar – you eat (singular)
• hann/hún/það borðar – he/she/it eats
• við borðum – we eat
• þið borðið – you eat (plural)
• þeir/þær/þau borða – they eat

So við borðum = we eat.

Saman means “together.”

In the clause:

við borðum saman á kvöldin

the basic order is:

• subject: við
• verb: borðum
• adverb: saman
• time expression: á kvöldin

So saman naturally comes after the verb and before the time phrase.
You will very often see adverbs like saman right after the verb in Icelandic main clauses.

Kvöld means “evening.” It is neuter.

The phrase á kvöldin literally means “on the evenings” and is in the definite plural:

• kvöld – evening / evenings
• kvöldin – the evenings (definite plural)

In Icelandic, repeated or habitual times are very often expressed with a definite plural time word:

• á morgnana – in the mornings
• á næturnar – at night(s)
• á laugardögum – on Saturdays
• á kvöldin – in the evenings

So á kvöldin is the standard idiomatic way to say “in the evenings” (regularly, as a habit).

Á kvöldið would more naturally mean “in the evening” referring to a specific evening, not a repeated habit.

Við borðum á kvöldin saman is grammatically possible, but it sounds less neutral and more marked in word order.

The most natural, neutral order is:

• Við borðum saman á kvöldin.

Placing saman right after the verb is the default.
Moving it to the end (á kvöldin saman) tends to give it extra emphasis or can sound slightly awkward unless you are stressing “together” in contrast to something else:

• like: “We eat in the evenings together (not separately).”

For everyday, neutral speech, keep:

• við borðum saman á kvöldin.

It corresponds most closely to:

• “One rule is that …”, or
• “One rule is this: that …”

The pattern is:

• Ein regla – one rule
• er – is
• sú – that one / the one (feminine, referring to regla)
• að … – that …

So you can think of it as:

“One rule is this (one): that we eat together in the evenings.”

Because regla is feminine, the words that describe it must also be feminine.

Two rules:

• tvær reglur – two rules (nominative plural)

Example:

• Tvær reglur eru þær að við borðum saman á kvöldin og að við slökkvum á símunum.
  = Two rules are that we eat together in the evenings and that we turn off our phones.

(þær is the feminine plural “they/those”.)

Another rule:

• önnur regla – another rule

Example:

• Önnur regla er sú að þú kemur á réttum tíma.
  = Another rule is that you arrive on time.