Aedilis mane forum intrat et a mercatoribus modestiam postulat.

Because aedilis is in the nominative singular, which is the case commonly used for the subject.

You can also tell from the verbs:

• intrat = he/she enters
• postulat = he/she demands

Both verbs are 3rd person singular, so they match a singular subject. Here that subject is aedilis.

Latin does not have articles like English the or a/an.

So:

• aedilis can mean an aedile or the aedile
• forum can mean a forum or the forum

You decide from the context which English translation sounds best.

Mane means in the morning or simply early in the day.

It is an adverb, not a noun. That means it describes when the action happens:

• Aedilis mane forum intrat = The aedile enters the forum in the morning

Because it is an adverb, it does not change its ending.

Forum is the direct object of intrat.

With verbs like enter, Latin often uses a direct object for the place being entered:

• forum intrat = he enters the forum

English sometimes uses a preposition in similar ideas, but Latin does not need one here.

Also, forum is a second-declension neuter noun, and in the singular its nominative and accusative are both forum, so the form looks the same in both cases.

Because intrat already means enters, and in Latin the place entered is commonly put directly in the accusative without in.

So:

• forum intrat = he enters the forum

If you used in foro, that would usually mean in the forum as a location, not into the forum as motion with enter.

Because modestiam is the thing being demanded, so it is the direct object of postulat.

• postulat = demands
• modestiam = moderation / proper behavior / self-restraint

So the structure is:

• something demanded = modestiam → accusative
• from whom it is demanded = a mercatoribus → ablative after a

Because Latin often expresses demanding something from someone as:

• aliquid ab aliquo postulare
  = to demand something from someone

So:

• modestiam postulat = he demands moderation
• a mercatoribus = from the merchants

After a/ab, the noun must be in the ablative case.

Here:

• mercatoribus = ablative plural
• meaning from the merchants

It is ablative plural.

The dictionary form is:

• mercator, mercatoris = merchant

This is a third-declension noun. Its ablative plural ends in -ibus:

• mercatoribus = from the merchants

That ending is a very useful clue.

Both a and ab mean from before the ablative.

In many textbooks you see ab as the basic form, but before consonants Latin can use either a or ab. Before vowels, ab is more common.

So here:

• a mercatoribus = from the merchants

is perfectly normal.

Because the sentence has one clear subject, aedilis, followed by two 3rd person singular verbs:

• intrat
• postulat

So the sentence works as:

• The aedile enters the forum
• and demands moderation from the merchants

Latin often leaves the subject stated only once when it applies to both verbs.

The exact order is less rigid in Latin than in English because the endings show the grammatical roles.

So even if the words were rearranged, the basic meaning could stay the same, for example:

• Mane aedilis forum intrat et modestiam a mercatoribus postulat

would mean essentially the same thing.

That said, word order still affects emphasis and style. The given sentence begins with aedilis, which gives the subject prominence.

They are both in the present tense.

• intrat = enters / is entering
• postulat = demands / is demanding

In a simple sentence like this, English usually translates them with the simple present:

• The aedile enters the forum in the morning and demands moderation from the merchants.

Modestia is a noun meaning something like:

• moderation
• self-restraint
• proper behavior
• discipline

In this sentence, it probably means that the aedile is demanding orderly or respectful conduct from the merchants.

So although the exact English word can vary, the grammar stays the same:

• modestiam = accusative singular, the thing being demanded.