Learn Croatian: Otac je jutros zaboravio ključ. - The father forgot the key this morning.

Even though this past tense form is called the l-participle, for masculine singular nouns like otac, the verb usually ends in -o. This is because a historical sound change in Croatian turned the final -l into an -o. If a woman had forgotten the key, the feminine form would keep the "l" and be zaboravila.

Why is the helper verb je separated from the main verb zaboravio?

In the Croatian past tense, short helper verbs like je are "clitics." A clitic is a lightweight word that strongly prefers the second position in a sentence. Here, Otac takes the first spot, so je must immediately follow it. The main verb zaboravio can then safely float later in the sentence, after the time word jutros.

Since the key (ključ) is the object being forgotten, shouldn't its ending change?

You are exactly right that ključ is the direct object and therefore in the accusative case. However, for masculine nouns that are "inanimate" (objects or concepts, not people or animals), the accusative form is identical to the dictionary form. Because a key is an object, ključ doesn't get an extra ending.

Is the word jutros (this morning) related to the word jutro (morning)?