Mihi ad forum eundum est, quia sal deest.

Because eundum est is a gerundive of obligation construction. In this construction, the person who has to do the action is usually put in the dative, not the nominative.

So:

• mihi = for me / by me / I
• mihi ad forum eundum est = literally something like to me it must be gone to the forum

In natural English, that becomes I must go to the forum or I have to go to the forum.

Eundum est means it must be gone or more naturally one must go.

This is formed from:

• eundum = the gerundive of eo, ire (to go)
• est = is

Together, they express necessity or obligation. In this sentence, it is best translated as I must go.

Because Latin is not simply saying to go; it is saying must go.

• ire is the infinitive: to go
• eundum est is a special construction meaning must be gone or has to be gone

So ad forum ire would just mean to go to the forum, while ad forum eundum est means it is necessary to go to the forum.

A Latin speaker could also say ad forum ire debeo (I ought to / must go to the forum), but this sentence uses the gerundive construction instead.

Because this is an impersonal use of the gerundive.

There is no nominative subject here for eundum to agree with, so Latin uses the neuter singular by default:

• eundum est = it must be gone

And est is singular for the same reason.
Mihi does not control agreement, because it is dative, not nominative.

It comes from the irregular verb eo, ire (to go).

This verb is unusual, and its gerundive uses the stem eund-, so:

• eundum = needing to be gone / to be gone

So even though it looks strange at first, it is the correct gerundive form of eo.

Ad takes the accusative when it means to or toward a place.

So:

• ad forum = to the forum

Forum here is accusative singular. It looks the same as the nominative because forum is a second-declension neuter noun, and neuter nominative and accusative singular have the same form.

Usually no, not with an ordinary noun like forum.

Latin often omits a preposition with names of towns, small islands, and a few special words such as domum. But with a common noun like forum, Latin normally uses a preposition:

• ad forum = to the forum

So ad forum is the normal and expected expression.

Deest comes from desum, deesse, which means to be lacking, to be missing, or to be absent.

The thing that is lacking is the subject of the verb, so it is in the nominative:

• sal deest = salt is lacking / there is no salt

That is why sal is nominative singular.

There can be a dative with deest if Latin wants to say to whom something is lacking:

• mihi sal deest = I lack salt / salt is lacking to me

But the dative does not have to be stated if it is clear from context. Here quia sal deest simply means because salt is lacking or because there is no salt.

No. Latin word order is fairly flexible because the endings show the grammar.

This order is natural:

• Mihi first gives prominence to me / for me
• ad forum eundum est gives the obligation
• quia sal deest adds the reason

A different order could still be grammatical, but this one is clear and idiomatic.