Учитель сказал, что перед публикой нужно говорить смелее.

Because что introduces a clause of reported speech, similar to that in English:

• Учитель сказал, что... = The teacher said that...

In Russian, что is very commonly used after verbs like:

• сказать = to say
• думать = to think
• знать = to know
• слышать = to hear

So the sentence is structured as:

• Учитель сказал = The teacher said
• что перед публикой нужно говорить смелее = that one should speak more confidently in front of an audience

Because the preposition перед usually takes the instrumental case when it means in front of / before.

So:

• публика = audience, public
• перед публикой = in front of the audience / before an audience

This is a standard pattern:

• перед домом = in front of the house
• перед уроком = before the lesson
• перед друзьями = in front of friends

Here the form публикой is the instrumental singular of публика.

Нужно is an impersonal word meaning it is necessary, one should, or it is needed.

In this sentence:

• нужно говорить смелее = one needs to speak more confidently / one should speak more boldly

There is no explicit subject like you or we. Russian often uses this kind of impersonal construction where English would use:

• you should
• one should
• it’s necessary to

So the sentence does not necessarily mean the teacher was speaking to one specific person. It can sound general: When speaking before an audience, you should be bolder/more confident.

Because говорить is imperfective, and here Russian is talking about the process or manner of speaking, not a single completed utterance.

Compare:

• говорить смелее = to speak more boldly/confidently
• сказать = to say something once, to utter something

In this sentence, the idea is not say one sentence boldly, but rather speak in a bolder way when addressing an audience. That is why говорить is the natural choice.

Смелее is the comparative form of смело (boldly, confidently) or related to смелый (bold, brave).

So:

• смело = boldly / confidently
• смелее = more boldly / more confidently

Russian often forms comparatives in one word, instead of using a separate word like more.

Examples:

• быстро = quickly → быстрее = more quickly / faster
• громко = loudly → громче = more loudly / louder
• смело = boldly → смелее = more boldly

In this sentence, смелее functions like an adverb, describing how someone should speak.

Literally, it comes from the idea of being bolder or braver, but in this context more confidently is often the most natural English translation.

With public speaking, говорить смелее usually suggests:

• don’t be timid
• speak with more confidence
• be bolder in front of people

So depending on context, it can be understood as:

• more boldly
• more confidently
• less timidly

Russian often leaves the subject unstated in impersonal constructions.

Here:

• нужно говорить смелее literally means something like it is necessary to speak more confidently

But in natural English, we often translate it as:

• you should speak more confidently
• one should speak more confidently

Russian does not need to say ты or вы here unless the speaker wants to be especially explicit. The statement is presented as a general recommendation.

It tells us the situation in which this advice applies: before an audience / in front of an audience.

So the structure is roughly:

• Учитель сказал = The teacher said
• что... нужно говорить смелее = that one should speak more confidently
• перед публикой = when in front of an audience

It modifies говорить: it tells us where / in what situation the speaking happens.

Yes, but the nuance is slightly different.

• перед публикой = in front of an audience; very literal and concrete
• на публике = in public / before other people; slightly broader

So:

• говорить перед публикой emphasizes speaking to an audience
• говорить на публике emphasizes speaking publicly

In your sentence, перед публикой fits very well because the idea is specifically public speaking in front of listeners.

Russian word order is more flexible than English word order. This sentence could be rearranged without changing the basic meaning much.

The given order:

• перед публикой нужно говорить смелее

naturally puts early focus on the setting: in front of an audience.

Other possible orders include:

• Нужно говорить смелее перед публикой.
• Смелее нужно говорить перед публикой.

These versions may sound slightly different in emphasis, but the core meaning stays the same.

Russian word order is often used to manage focus, emphasis, and flow, not just grammar.

Because сказал is the past tense masculine singular form of сказать.

Russian past tense agrees with the gender and number of the subject:

• он сказал = he said
• она сказала = she said
• оно сказало = it said
• они сказали = they said

Since учитель usually refers to a male teacher, the sentence uses сказал.

If it were a female teacher, it would be:

• Учительница сказала, что перед публикой нужно говорить смелее.

In normal usage, учитель is the masculine noun male teacher.

So this sentence most naturally means a male teacher said...

If you want to refer clearly to a female teacher, Russian usually uses:

• учительница = female teacher

That said, in some modern contexts people may discuss profession words more flexibly, but for learners the standard distinction is:

• учитель = male teacher
• учительница = female teacher

Not in this sentence. Here it is part of reported speech after сказал, что...

So the teacher’s idea is being reported indirectly:

• Учитель сказал, что... = The teacher said that...

A direct command would look different, for example:

• Говори смелее перед публикой! = Speak more confidently in front of an audience!
• Говорите смелее перед публикой! = Speak more confidently in front of an audience! (polite/plural)

So your sentence is indirect speech, not a direct imperative.