Терминологию добавили в глоссарий.

Because it is the direct object of the verb добавили (added).

The base form is терминология. Since it is a feminine noun ending in -я, its accusative singular form is терминологию.

• nominative: терминология
• accusative: терминологию

So the sentence structure is basically:

• добавили что? → терминологию

That is why Russian uses the accusative here.

This is a very common Russian pattern. The verb is in the 3rd person plural past form, but no subject is stated. It means something like:

• they added the terminology to the glossary
• or more naturally in English: the terminology was added to the glossary
• or someone added the terminology to the glossary

Russian often uses this indefinite-personal construction when the doer is unknown, unimportant, or simply not mentioned.

So добавили does not necessarily mean a specific group of people. It can just mean it was added in a general sense.

Because в can take either:

• accusative for motion/direction (into, to)
• prepositional for location (in, inside)

Here, the idea is adding something into the glossary, so Russian uses в + accusative:

• в глоссарий = into/to the glossary

Compare:

• добавили в глоссарий = added to the glossary
• в глоссарии есть термин = there is a term in the glossary

So the sentence focuses on the result of movement/addition into the glossary, not just location.

In Russian, the verb добавить commonly uses в + accusative when something is added into a list, text, file, glossary, document, etc.

So:

• добавить в список = add to the list
• добавить в словарь = add to the dictionary
• добавить в глоссарий = add to the glossary

Even though English says to, Russian often uses в because the image is putting something into a container or collection.

Добавили is the perfective past tense of добавить.

Perfective is used because the sentence presents the action as completed:

• the terminology was added
• the action is done, viewed as a whole

The imperfective partner is добавляли, which would suggest process, repetition, or background context:

• Терминологию добавляли в глоссарий весь день. = They were adding terminology to the glossary all day.

But добавили means the addition happened and was completed.

The dictionary form is добавить.

Here is the breakdown:

• infinitive: добавить = to add
• past masculine singular: добавил
• past feminine singular: добавила
• past neuter singular: добавило
• past plural: добавили

So добавили is simply the past plural form.

Russian word order is flexible, so starting with терминологию is completely normal.

Терминологию добавили в глоссарий puts a little more attention on the terminology itself. It can sound like:

• As for the terminology, they added it to the glossary
• The terminology was added to the glossary

A more neutral order is also possible:

• Добавили терминологию в глоссарий

But the original order is natural and often preferred when the object is the main topic or focus of the statement.

Grammatically, терминология is singular.

It is a collective/abstract noun, like English terminology. Even though it refers to a set of terms, the noun itself is singular.

So:

• терминология = terminology
• not terminologies in the usual sense

That is why the accusative form is singular: терминологию.

Yes, very naturally.

Although Russian does not use a passive form here, English often prefers one. So all of these can work depending on context:

• They added the terminology to the glossary.
• The terminology was added to the glossary.
• Someone added the terminology to the glossary.

In Russian, the active plural form without a subject often covers the same function that English expresses with a passive.

Yes. A more explicitly passive version would be:

• Терминология была добавлена в глоссарий.

This means the same thing: The terminology was added to the glossary.

The difference is mostly stylistic:

• Терминологию добавили в глоссарий = more everyday, common, natural in many contexts
• Терминология была добавлена в глоссарий = more formal, more explicitly passive

Russian often prefers the first version where English might choose the passive.

Russian has no articles like English the and a/an.

So Терминологию добавили в глоссарий can mean:

• The terminology was added to the glossary
• Terminology was added to a/the glossary

Which one is intended depends on context.

Russian speakers understand definiteness from the situation, word order, and context rather than from articles.

Yes. Глоссарий is a masculine noun.

Its nominative singular form is глоссарий. After в with the meaning of direction, it takes the accusative. For masculine inanimate nouns, the accusative singular is the same as the nominative, so it stays:

• глоссарий → в глоссарий

That is why the form does not change visibly here.

A rough pronunciation guide is:

ter-mee-na-LO-gee-yu da-BA-vee-lee v glos-SA-riy

A more careful breakdown:

• Терминологию → teer-mee-na-LO-gee-yu
• добавили → da-BA-vee-lee
• в глоссарий → v glos-SA-riy

The main stress falls on:

• терминологию
• добавили
• глоссарий

If you want to sound natural, it helps to practice the whole sentence as one rhythm group:

Терминологию добавили в глоссарий.