Подсекция началась утром.

In the past tense, Russian verbs agree with the subject in gender and number. Since подсекция is feminine singular, the verb is also feminine singular: началась.

Compare:

• секция началась — feminine singular
• урок начался — masculine singular
• собрание началось — neuter singular
• занятия начались — plural

The -сь is the reflexive/postfix form of -ся. The verb here is начаться, not начать.

• начать = to start something
• начаться = to begin, to start (of an event/process itself)

So подсекция началась means the subsection/session began, not that someone started it in a direct active way.

Началась is perfective, so it presents the beginning as a single completed event: it started.

Начиналась is imperfective, and would usually suggest:

• a repeated/habitual action in context, or
• a background/process view

So for a simple statement that something began at a certain time, началась is the normal choice.

Утром is the instrumental case of утро, but in this expression it works adverbially and means in the morning. Russian often uses bare instrumental forms of parts of the day this way:

• утром — in the morning
• днём — in the daytime / during the day
• вечером — in the evening
• ночью — at night

So this is a very common time expression, not a random case choice.

By itself, утром can mean either, depending on context. Very often it is just in the morning.

If Russian wants to make this morning especially clear, it can say:

• сегодня утром — this morning

So context tells you how specific it is.

Yes. Russian word order is flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• Подсекция началась утром. — neutral
• Утром началась подсекция. — emphasizes утром
• Началась утром подсекция. — possible, but more marked/stylistic

The most natural version depends on what information is new or important in the context.

You could, and it is grammatically correct, but it sounds more formal, bookish, or passive. Russian usually prefers началась for events like meetings, classes, sessions, conferences, and similar things.

So:

• Подсекция началась утром. — natural, normal
• Подсекция была начата утром. — more formal/passive, often less natural in everyday use

It depends on context. Подсекция can refer to a subsection in an organizational or structural sense, and in some contexts it can also mean a smaller division of a section, including at conferences or organized events.

So the grammar of the sentence does not change, but the exact real-world meaning of подсекция depends on what kind of thing is being discussed.