Learn Icelandic: Ég hjálpaði Jóni í gær.

The verb hjálpa (to help) always forces its object into the dative case, rather than the more common accusative. Proper names in Icelandic decline just like regular nouns, so the standard nominative form Jón changes to its dative form, Jóni.

Does every masculine name just add an -i in the dative case like Jóni?

Not all of them, though it is very common for strong masculine names ending in a consonant (like Jón, Haraldur, or Stefán). However, if a masculine name is "weak" and already ends in -i in the nominative (like Bogi or Kári), it usually changes to an -a in the dative (becoming Boga or Kára).

How do we know the past tense of hjálpa is hjálpaði?

Hjálpa belongs to the largest and most regular group of weak verbs in Icelandic (Class 4). These verbs end in -a in the infinitive and form their past tense by adding -ði to the stem. For the first-person singular (ég), this predictably results in the -aði ending.

Why do we use the preposition í for "yesterday" (í gær)?