Learn Icelandic: Strákurinn man regluna. - The boy remembers the rule.

Man is from the verb muna (to remember). It belongs to a special category of verbs that conjugate exactly like the modal verb mega (may) that you are focusing on in this lesson. For these verbs, the 'I' and 'he/she/it' forms in the present tense don't get an -r ending. So, 'the boy remembers' is simply strákurinn man.

Why is the word for 'the rule' spelled regluna here instead of reglan?

The verb muna forces the thing being remembered into the accusative case. The basic feminine word for 'a rule' is regla. If it were the subject ('the rule is...'), it would be reglan, but because it is the object receiving the action here, the definite ending changes to -na, giving us regluna.

Is the word man related to the English word 'man' or the Icelandic word maður?

Not at all, it is just a 'false friend'! Here, man is strictly the verb meaning 'remembers'. The Icelandic word for a man or person is maður (which becomes mann in the accusative case).

How does the ending on strákurinn work?

Strákur is a masculine noun meaning 'boy' in the nominative (subject) case. To say 'the boy', you add the masculine definite article suffix -inn to the end, resulting in strákurinn.