Bos cornu, quo portam paene tangit, tam magnum habet ut puer timeat.

Here quo is the ablative singular of the relative pronoun qui, quae, quod. It refers back to cornu, so it agrees with cornu in gender and number: both are neuter singular.

But its case comes from its job inside the relative clause, not from the antecedent. In the clause quo portam paene tangit, the sense is with which it almost touches the gate, so Latin uses the ablative of means/instrument: quo = with which.

It refers to cornu, the horn.

You can tell this both from the meaning and from the grammar:

• cornu is neuter singular
• quo is also neuter singular
• portam is feminine singular, so quo cannot refer to it

So the idea is that the ox almost touches the gate with its horn.

The understood subject is bos.

Latin often leaves the subject unstated when it is clear from context. The verb tangit is 3rd person singular, and here the natural subject is the same as in the main clause: the ox.

So the clause means: with which the ox almost touches the gate.

Because tangere takes a direct object in the accusative. The thing being touched is put in the accusative, so:

• porta = gate
• portam = gate as direct object

Thus portam tangit means touches the gate.

Paene is an adverb meaning almost or nearly, and it modifies tangit.

So:

• portam tangit = touches the gate
• portam paene tangit = almost touches the gate

It does not describe the gate; it describes how close the action of touching comes to happening.

This is the standard Latin result construction:

tam + adjective/adverb + ut + subjunctive
= so ... that ...

So here:

• tam magnum = so large
• ut puer timeat = that the boy is afraid / that the boy gets frightened

The whole idea is: the ox has a horn so large that the boy is afraid.

Because ut puer timeat is a result clause, and result clauses in Latin normally take the subjunctive.

So timeat is not subjunctive because the action is doubtful; it is subjunctive because the grammar of the construction requires it after tam ... ut.

Since the main verb habet is present tense, the present subjunctive timeat is the normal tense here.

Tam magnum agrees with cornu.

Both are:

• neuter
• singular
• accusative

Latin allows words that belong together to be separated by other material. So even though the relative clause quo portam paene tangit comes in between, cornu ... tam magnum still goes together.

A learner-friendly way to see the structure is:

Bos cornu tam magnum habet ut puer timeat,
with quo portam paene tangit inserted as extra information about cornu.

Because that is perfectly normal Latin. Latin often uses habeo to express possession, including possession of body parts:

• bos cornu habet = the ox has a horn

English might sometimes prefer something like its horn is very large, but Latin is happy to say the ox has a horn so large... There is nothing unusual or unidiomatic about that.

Latin has no articles. It does not have separate words that exactly equal English the or a/an.

So:

• bos can mean an ox or the ox
• cornu can mean a horn or the horn
• portam can mean a gate or the gate

Context, or the translation already given to the learner, tells you which is best in English.

By English standards, yes; by Latin standards, not especially.

Latin word order is much freer because the endings show how words fit together. Here the sentence places:

• the main subject first: Bos
• the object next: cornu
• then a relative clause describing that horn: quo portam paene tangit
• then the adjective phrase completing the description: tam magnum
• and finally the result clause: ut puer timeat

So the order is shaped more by style and emphasis than by strict grammatical necessity.