Miles pontem subit.

Because Latin shows each noun’s job by its case ending.

Miles is in the nominative singular, which is the case normally used for the subject.

Pontem is in the accusative singular, which is the case normally used for the direct object.

So even if the word order changed, the endings would still tell you who is doing the action and what is being affected by it.

Yes, miles is singular.

It is the nominative singular of miles, militis, a masculine third-declension noun. In Latin, an ending with -s does not automatically mean plural. That is an English habit, not a Latin rule.

The nominative plural is milites.

So: miles = one soldier
milites = soldiers

Because pontem is the accusative singular form, and this noun is the direct object.

The dictionary form is pons, which is nominative singular. Its full dictionary entry is pons, pontis.

Like many third-declension nouns, it changes its form noticeably: pons = nominative singular
pontem = accusative singular

So the change from pons to pontem is a normal third-declension pattern.

Subit is third person singular, present active indicative.

That means it is the form used for he/she/it in the present tense.

The ending -it here is part of the present-tense form of this verb, and the -t specifically marks third person singular.

So grammatically, subit means he/she/it ...s or is ...ing, depending on how you express the Latin in English.

It comes from subeo, subire.

This is a compound of sub- plus eo, ire, the verb to go.

Because eo is irregular, its compounds keep some of that irregularity: subeo = I ...
subis = you ...
subit = he/she/it ...

So if you were trying to look up subit in a dictionary, you would look for subeo.

Because Latin often builds that kind of spatial idea directly into the verb.

In subit, the prefix sub- contributes the idea of movement connected with under or up to, depending on context and meaning.

English often uses a simple verb plus a separate preposition, but Latin very often prefers a compound verb instead.

So this sentence does not need a separate preposition to express that basic idea.

Latin has no articles.

That means miles can mean a soldier or the soldier, and pontem can mean a bridge or the bridge. The context tells you which is meant.

When Latin is translated into English, the translator has to choose the most natural article for the context.

Yes, it could be arranged differently.

Latin word order is much more flexible than English word order because the endings already show each word’s role. Still, putting the verb at or near the end is very common and often feels neutral or straightforward.

So Miles pontem subit is a very normal Latin order. But other orders are possible, usually with a change in emphasis rather than a complete change in basic meaning.

Because the verb ending already tells you the person and number.

In subit, the ending shows third person singular, so Latin does not need a separate subject pronoun unless it wants extra emphasis.

Also, the subject noun miles is already present, so adding a pronoun would usually be unnecessary.

You would look them up under their basic dictionary forms:

miles → miles, militis
pontem → pons, pontis
subit → subeo, subire

This is an important Latin skill: the form you see in a sentence is often not the form listed in the dictionary, especially for nouns in oblique cases and for finite verb forms.