Coquus modum aquae addit, ne panis nimis siccus sit.

Because coquus is the subject of the main verb addit.

• coquus = the cook
• addit = adds

In Latin, the subject is usually in the nominative case, and coquus is nominative singular. If it were coquum, that would be accusative, which would usually mark a direct object instead.

Modum aquae means a measure/amount of water.

Here is the grammar:

• modum is the accusative singular of modus
• aquae is the genitive singular of aqua

So literally it means a measure of water.

This is a very common Latin pattern:

• a noun in one case
• followed by a genitive showing of ...

So modum aquae = an amount of water.

Because it is the direct object of addit.

The verb addit means adds, and the thing being added is modum aquae. In Latin, the direct object normally goes in the accusative case, so modus becomes modum.

Because aquae is not a second direct object. It depends on modum and tells you what kind of measure it is: a measure of water.

So:

• modum = the object actually being added
• aquae = of water, describing modum

That is why aquae is genitive, not accusative.

Here ne introduces a negative purpose clause.

So:

• non usually just means not
• ne often means so that ... not or lest

In this sentence, ne panis nimis siccus sit means something like:

• so that the bread may not be too dry
• lest the bread be too dry

So ne is not just a simple negative word here; it signals the purpose of the cook’s action.

Because after ne in a purpose clause, Latin normally uses the subjunctive mood.

• est = indicative, is
• sit = present subjunctive, may be / be

So ne panis nimis siccus sit is not just stating a fact. It expresses the cook’s aim or intention:

• he adds water so that the bread may not be too dry

That is why sit is correct here.

Because panis is the subject of sit in the subordinate clause.

In ne panis nimis siccus sit:

• panis = the bread
• siccus = dry
• sit = may be

So the structure is:

• the bread = subject
• may be = verb
• too dry = predicate adjective

Since panis is the subject, it is in the nominative case.

Because siccus agrees with panis.

Panis is a masculine singular noun, so the adjective describing it must also be masculine singular:

• panis = masculine singular
• siccus = masculine singular

If the noun were neuter, then siccum might be used instead. But here the adjective has to match panis.

Nimis is an adverb meaning too, excessively, or overly.

It modifies siccus:

• siccus = dry
• nimis siccus = too dry

Even though siccus is an adjective, Latin often uses adverbs like nimis to strengthen it, just as English says too dry.

Because that is a very natural Latin word order.

The sentence first gives the main action:

• Coquus modum aquae addit = The cook adds an amount of water

Then it gives the purpose:

• ne panis nimis siccus sit = so that the bread may not be too dry

Latin word order is more flexible than English, but putting the purpose clause after the main clause is completely normal.

Yes, often they could.

Latin relies much more on endings than on fixed word order, so a sentence like this can often be rearranged without changing the basic meaning. For example, the cases still show what each word is doing:

• coquus is still the subject
• modum is still the object
• aquae is still genitive
• panis is still the subject of sit

However, changing the word order can change the emphasis. The given order is natural and clear, especially because it places the purpose clause neatly at the end.

Sit is present subjunctive.

In a purpose clause after a present-tense main verb like addit, the present subjunctive is exactly what you expect. It does not simply mean present time in the same way as est. Instead, it expresses the intended result or goal of the action.

So here the sense is:

• the cook adds water now
• with the purpose that the bread not be too dry

It is a purpose clause.

The key clue is ne with the subjunctive, giving the reason or aim behind the action:

• He adds water in order that the bread not be too dry.

A result clause would usually be introduced differently, often with ut or ut non after a word suggesting degree or consequence. Here the sentence is clearly about intention, so purpose clause is the best label.