Learn Russian: Мальчик любит фонтан.

You are right that фонтан is the direct object here, so it is in the Accusative case. However, in Russian, masculine nouns that are inanimate (objects and places, not people or animals) look exactly the same in the Accusative case as they do in their basic dictionary form. If the boy loved a person or animal, the object's ending would change.

Why does the verb end in -ит (любит)?

The dictionary form of this verb is любить (to love). It belongs to a group of verbs that use -и- in their endings. When the subject is a he, she, or it (like мальчик), the correct ending to use is -ит.

If I change the word order to Фонтан любит мальчик, does it change the meaning?

No, it still means "The boy loves the fountain." Russian uses word endings rather than word order to show who is doing the action. Since мальчик is in the subject form, we know the boy is doing the loving. To say "The fountain loves the boy," you would have to change the boy to the object form: Фонтан любит мальчика.

What does the soft sign (ь) do in the word мальчик?

The soft sign itself has no sound, but it changes how you pronounce the consonant right before it. It tells you to pronounce the л (l) softly, making it sound a bit like the "lli" in the English word "million."