Learn Icelandic: Ég kaupi síma núna. - I am buying a telephone now.

sími is the basic dictionary form. In this sentence, the telephone is the direct object (the thing you are buying), so it changes to the accusative case. For masculine nouns that end in -i, the accusative form changes the ending to -a, giving us síma.

Where is the word for 'a' in 'a telephone'?

Icelandic does not use words for 'a' or 'an' (indefinite articles). The bare noun síma translates to 'a telephone' all on its own.

Does ég kaupi mean 'I buy' or 'I am buying'?

It can mean either! Icelandic does not have a separate continuous tense like English does. Ég kaupi translates to 'I buy' or 'I am buying' depending on what makes sense. The word núna (now) makes it clear that the action is happening right now.

Can I change the word order and start the sentence with núna?

Yes, but if you move núna to the front, the verb and subject must swap places. You would say Núna kaupi ég síma. This happens because of the V2 rule in Icelandic, which forces the verb to remain the second piece of information in the sentence.