Breakdown of Í gær gleymdi ég að hringja í hann.
ég
I
hann
he
að
to (infinitive marker)
í
in / into / to (before the hour, in telling time) / per (in frequency expressions)
Questions & Answers about Í gær gleymdi ég að hringja í hann.
Why does gleyma become gleymdi here?
Gleyma (to forget) is a weak verb. To make it past tense for ég (I), we drop the infinitive ending -a and add the past tense suffix -di. This makes gleymdi (I forgot).
Why is the word order gleymdi ég and not ég gleymdi?
What is the purpose of að before hringja?
The word að is the infinitive marker, just like the word 'to' in English. When you say 'forgot to call', the second verb stays in its dictionary form and gets an að in front of it: að hringja.
Why do we say hringja í hann instead of just hringja hann?
In Icelandic, the verb hringja (to ring or call) requires the preposition í (in or into) when calling someone on the phone. You literally say that you 'ring into' him, which is why we use í hann.