gōnggào shuō zhè shì yí gè gōngyì huódòng.

The sentence breaks down like this:

• 公告 = announcement / notice
• 说 = says / states
• 这 = this
• 是 = is
• 一 个 = one + measure word, together meaning a / one
• 公益活动 = public welfare activity / charity event /公益 event

So the structure is:

公告 + 说 + 这 + 是 + 一个 + 公益活动

A natural grammar explanation is:

The announcement says + this + is + a public-welfare activity.

In Mandarin, this kind of structure is very normal:
A says B → A 说 B

So 公告说这是一个公益活动 literally means:

The announcement says this is a public welfare activity.

This is because of a very common tone change rule.

Normally, 一 is first tone: yī.

But when 一 comes before a fourth-tone syllable, it changes to second tone: yí.

Here:

• 个 is pronounced gè (fourth tone)
• so 一 个 becomes yí gè

Some useful patterns:

• 一 + 4th tone → yí
  • 一个 → yí gè
  • 一天 is actually yì tiān? No, because 天 is first tone, so see below.
• 一 + non-4th tone → usually yì
  • 一天 → yì tiān
  • 一年 → yì nián

When 一 is said by itself, or emphasized as the number one, it stays yī.

So in this sentence, 一 is pronounced yí because it comes before 个, which is fourth tone.

个 is a measure word (also called a classifier).

In Mandarin, when you count nouns or say a/an + noun, you usually need:

number/demonstrative + measure word + noun

So:

• 一个活动 = an activity
• 这个活动 = this activity
• 那个活动 = that activity

Here:

• 一 = one / a
• 个 = measure word
• 活动 = activity

So 一个活动 means an activity.

Why 个? Because 个 is the most common general measure word, and it works with many nouns, including 活动.

Yes, sometimes you can, but the meaning and tone can feel different.

Compare these:

• 这是一个公益活动。
  = This is a public welfare activity.
  This sounds like ordinary identification: you are classifying it as one kind of activity.

• 这是公益活动。
  = This is public welfare activity / This belongs to the category of public welfare activities.
  This can sound more formal, more categorical, or less conversational.

In everyday speech, 这是一个公益活动 is often the more natural and learner-friendly version.

So:

• with 一个 → very common, natural for introducing or identifying something
• without 一个 → possible, but often more abstract or formal

公告说 means the announcement says or the notice states.

A very common Mandarin pattern is:

X 说 ... = X says ...

So here:

• 公告 is the subject
• 说 is the verb
• the rest of the sentence is what the announcement says

In English, we sometimes use inanimate subjects too:

• The sign says ...
• The notice says ...
• The report says ...

Mandarin does the same thing. So 公告说 is completely natural.

Also, 说 here is not only literal spoken speech. It can also mean:

• says
• states
• mentions

So 公告说 does not mean the announcement literally spoke out loud. It just means the announcement stated that information.

Mandarin does not have articles like a, an, and the in the same way English does.

So 公告 can mean:

• an announcement
• the announcement
• announcement(s)

The exact meaning depends on context.

In this sentence, English naturally translates it as the announcement because both speaker and listener probably know which announcement is being referred to.

If you want to be more specific in Mandarin, you could say:

• 这个公告 = this announcement
• 那个公告 = that announcement

But very often, just 公告 is enough.

So the lack of the is normal Mandarin grammar.

Literally, 这 means this.

So:

• 这是... = This is...

In English, depending on context, we might also translate it as It is... if that sounds more natural.

But grammatically, the Chinese is using this, not a separate word meaning it.

A few related forms:

• 这 = this
• 那 = that
• 这个 = this one / this thing
• 那个 = that one / that thing

In this sentence, 这 refers to whatever the announcement is talking about — for example, an event, campaign, or activity.

So the sentence is literally:
The announcement says this is a public welfare activity.

是 is the verb to be when identifying or equating something.

Here the sentence is:

这 是 一个公益活动。
= This is a public welfare activity.

You use 是 because you are saying that this belongs to the category 公益活动.

This is similar to:

• 这是老师。 = This is a teacher.
• 这是问题。 = This is a problem.
• 这是活动。 = This is an activity.

A useful learner note: in Mandarin, 是 is often used for A is B type sentences, especially when identifying nouns.

公益 relates to public welfare, the public good, or charitable/social benefit.

In 公益活动, it functions like a modifier describing the type of activity:

• 公益活动 = public welfare activity
• often translated naturally as:
  • charity event
  • nonprofit activity
  • public-benefit activity

So in this phrase, 公益 acts somewhat like an adjective in English, even though Chinese word classes are often more flexible.

You can think of it as:

• 公益 = public welfare /公益
• 活动 = activity
• together: an activity for the public good

Depending on context, English translations may vary. Charity event is often a smooth translation, but public welfare activity is closer to the original wording.

Mandarin usually follows a straightforward subject + verb + object/content order.

Here:

• 公告 = subject
• 说 = verb
• 这是一个公益活动 = the content of what is said

So it is:

Announcement + says + this is a public welfare activity

This is very similar to English:

The announcement says this is a public welfare activity.

You cannot say something like 公告这说是一个公益活动 because that breaks the normal structure.

A very common Mandarin pattern is:

• 老师说他今天不来。
  = The teacher said he is not coming today.
• 新闻说天气会变冷。
  = The news says the weather will get colder.
• 公告说这是一个公益活动。
  = The announcement says this is a public welfare activity.

So the sentence order is quite standard.

活动 is pronounced huó dòng.

Tones:

• 活 = second tone (huó)
• 动 = fourth tone (dòng)

A rough pronunciation guide:

• huó sounds a bit like hwor with a rising tone
• dòng sounds like dong with a falling tone

Also, huódòng is a very common word meaning:

• activity
• event
• campaign
• function

So this is a useful word to remember as a whole.

In this sentence, the full phrase 公益活动 is:

• gōngyì = public welfare
• huódòng = activity

Together: gōngyì huódòng

In this sentence, 公告 is a noun meaning announcement or public notice.

So:

• 公告说... = The announcement says...

However, in Chinese, 公告 can also appear in ways that feel verbal in some contexts, especially in formal language, where it can mean to announce publicly.

But for this sentence, the easiest and most accurate understanding is:

• 公告 = noun
• 说 = verb

So do not read this sentence as announce says...
Read it as the announcement says...

Yes, depending on style.

Possible alternatives include:

• 公告称这是一个公益活动。
  More formal; 称 means states / claims
• 公告写着这是一个公益活动。
  Means the notice says/writes that this is...
• 公告表示这是一个公益活动。
  More formal; indicates / states

But 说 is simple, common, and very natural for learners.

So:

• 说 = everyday and flexible
• 称 / 表示 = more formal, often seen in news or official language
• 写着 = emphasizes written wording

In your sentence, 说 is a very normal choice.