Haec medicina ventri eius bene medetur.

Because medeor is a deponent verb. Deponent verbs use passive-looking forms, but they have an active meaning.

So:

• medetur looks like a passive form
• but it means he/she/it heals, treats, or is good for

Here it is:

• 3rd person singular
• present tense
• indicative mood

So medetur means it heals / it is beneficial to.

Because medeor commonly takes the dative of the person or body part being treated.

So Latin says something more like:

• this medicine heals the stomach but structurally:
• this medicine is good for / gives healing to the stomach

That is why we get:

• ventri = to the stomach / for the stomach

It is the dative singular of venter, ventris.

The subject is haec medicina.

More specifically:

• medicina is the noun
• haec modifies it

Both are nominative feminine singular, so they go together and mean this medicine.

Since medetur is singular, that matches the singular subject haec medicina.

Because medicina is a feminine singular noun.

The demonstrative hic, haec, hoc must agree with the noun it modifies in:

• gender
• number
• case

So with medicina we need:

• haec = nominative feminine singular

That gives haec medicina = this medicine.

Eius means his, her, or its, depending on the context.

In this sentence it modifies ventri, so:

• ventri eius = his stomach, her stomach, or its stomach

Grammatically, eius is:

• genitive singular
• from is, ea, id

It does not show gender by itself in this form, so only the context tells you whether it means his, her, or its.

Because suus is a reflexive possessive adjective: it refers back to the subject of the clause.

But the subject here is haec medicina. The stomach does not belong to the medicine. It belongs to someone else already understood from context.

So Latin uses eius for his/her/its referring to someone other than the subject.

That is why ventri eius is correct here.

Bene is an adverb, meaning well.

It modifies medetur, so the idea is:

• heals well
• works well for
• is good for

It does not describe the noun medicina. If it described a noun, Latin would need an adjective, not the adverb bene.

Latin word order is more flexible than English word order because the endings show the grammatical relationships.

So:

• Haec medicina ventri eius bene medetur is the natural sentence given here, but other orders are possible, such as:
• Haec medicina bene ventri eius medetur
• Ventri eius haec medicina bene medetur

The meaning stays basically the same because:

• haec medicina is nominative, so it is the subject
• ventri is dative, so it depends on medetur
• eius goes with ventri
• bene modifies the verb

However, changing the order can change the emphasis.

Medicina is a first-declension feminine noun.

In this sentence it is:

• nominative singular
• the subject of medetur

Its basic meaning can be medicine, remedy, or treatment, depending on context.

Ventri is the dative singular of venter, ventris, a third-declension masculine noun.

Its basic meaning is:

• belly
• stomach
• sometimes womb in other contexts

Here it means stomach/belly, and the dative is used because of the verb medetur.

Yes, sometimes that is a good way to understand it.

With the dative, medeor can suggest:

• heal
• treat
• be good for
• help medically

So depending on context, the sentence could be understood as something like:

• This medicine heals his stomach
• This medicine is good for his stomach
• This medicine helps his stomach

The exact English wording depends on how literal or natural you want the translation to be.