“Venis” in indicativo est, sed “veni” in imperativo est.

Venis is the second-person singular present active indicative of venire.

So it is the form used when you are making a statement or asking a question to one person, not giving a command. In grammatical terms, the important label here is indicative.

Here veni is the second-person singular present active imperative of venire.

That means it is the command form addressed to one person: come! The sentence is pointing out that this form belongs to the imperative mood.

Because both forms come from the same verb, venire, and both are singular forms addressed to one person.

The difference is the mood:

• venis = indicative
• veni = imperative

So the contrast is not about the basic idea of the verb, but about how the speaker is using it:

• statement/question form: venis
• command form: veni

Because venire is a fourth-conjugation verb.

In Latin, fourth-conjugation verbs normally form the singular imperative with -i:

• audi from audire
• veni from venire

So veni is exactly the regular imperative you would expect. It is not irregular in that respect.

They are grammatical labels meaning in the indicative and in the imperative.

More literally, indicativo and imperativo are ablative singular forms after in. Latin grammar explanations often use this pattern when naming moods.

You can think of an understood noun such as modo:

• in modo indicativo
• in modo imperativo

Very often modo is left out, because the meaning is clear without it.

Yes. Veni can also be the first-person singular perfect active indicative of venire, meaning I came.

So by itself, veni is ambiguous:

• veni! = come!
• veni = I came

Latin normally leaves this to context. In your sentence, the phrase in imperativo tells you explicitly that veni is being discussed as an imperative form.

Not completely.

With macrons, you would write:

• venīs
• venī

But that still does not remove the bigger ambiguity, because the imperative venī and the perfect venī are spelled the same way.

So even with careful marking of vowel length, you still need context to know whether venī means come! or I came.

Normally, no.

Venis is an indicative form, so its normal job is to make a statement or ask a question. If you want to give a direct command to one person, Latin normally uses veni, not venis.

So the sentence is contrasting the normal statement/question form with the normal command form.

The plural imperative is venite.

So the present imperative of venire is:

• singular: veni
• plural: venite

That is a very useful pair to memorize.

The dictionary form is venire, meaning the infinitive to come.

From that verb you get forms such as:

• venis = present indicative, second singular
• veni = present imperative, second singular

So both forms in the sentence are built from the same verb, venire.