zìcóng tā zuò zhìyuànzhě yǐhòu, chángcháng cānjiā gōngyì huódòng.

This is a very common time pattern in Mandarin.

• 自从 introduces a starting point
• 以后 marks the period after that point

Together, 自从...以后 means ever since... or since...

So:

• 自从她做志愿者以后 = ever since she started being a volunteer / ever since she became a volunteer

Using both is normal and sounds natural. Mandarin often uses this kind of paired structure for clarity.

Often, yes.

You may hear:

• 自从她做志愿者，常常参加公益活动。
• 她做志愿者以后，常常参加公益活动。

But there is a slight difference:

• 自从 strongly emphasizes since / ever since
• 以后 by itself means after
• 自从...以后 gives the clearest from that time until now feeling

So the full version is very natural, especially when describing a continuing change in behavior.

Because Mandarin often drops a repeated subject when it is already clear.

In this sentence, the subject in both parts is 她, so the second 她 is omitted:

• 自从她做志愿者以后，（她）常常参加公益活动。

This is extremely common in Chinese. English usually repeats the subject, but Chinese often does not if there is no confusion.

In Mandarin, 做 can be used with jobs, roles, or identities to mean work as, serve as, or sometimes be.

Examples:

• 做老师 = work as a teacher
• 做司机 = work as a driver
• 做志愿者 = serve as a volunteer / be a volunteer

So 做 here does not mean make. It is part of a common pattern: 做 + role/job.

It can suggest either one depending on context.

In this sentence, because of 自从...以后, English often translates it naturally as:

• since she became a volunteer or
• since she started volunteering

But 做志愿者 itself more literally means to be / serve as a volunteer.

If you want to make the idea of becoming clearer, Mandarin often uses:

• 成为志愿者
• 当上志愿者 (more colloquial in some contexts)

So the original sentence is fine, but it is slightly broader than a strict became.

常常 is a frequency adverb, like often.

In Mandarin, frequency words such as these usually come before the main verb:

• 她常常参加活动。
• 我经常去那儿。
• 他们总是很忙。

So in this sentence:

• 常常参加 = often participate in

That word order is standard Chinese grammar.

Because this sentence describes a habitual, ongoing situation, not just one completed event.

• 常常参加 means she often participates
• This is a repeated action, so 了 is usually not needed

Also, Mandarin does not mark tense the way English does. Time is often shown through context and time expressions such as 自从...以后.

If you added 了 after 参加, it would sound more like specific completed instances, which does not fit as well with 常常.

That meaning comes from the combination of:

• 自从...以后 = starting from some point in the past
• 常常 = repeated/habitual action

Together they show this idea:

• from the time she started being a volunteer up to now, she often participates in public-service activities

So even without English-style tense endings, the sentence clearly suggests an ongoing pattern.

公益活动 is a noun phrase:

• 公益 = public welfare / charitable / for the public good
• 活动 = activity / event

So 公益 modifies 活动, giving:

• 公益活动 = public-welfare activities / charity activities / community service activities

This is a very common Chinese pattern: one word modifies the noun that follows it.

Examples:

• 文化活动 = cultural activities
• 体育活动 = sports activities
• 环保活动 = environmental activities

Because 参加 and 加入 are not used in exactly the same way.

• 参加 = participate in, take part in
• 加入 = join, become a member of

Here, the object is 公益活动 which is an activity, so 参加 is the correct verb:

• 参加公益活动 = participate in public-welfare activities

If the object were an organization or group, 加入 might be better:

• 加入志愿者协会 = join a volunteer association

Yes, that is also natural.

Both of these are acceptable:

• 自从她做志愿者以后，常常参加公益活动。
• 她自从做志愿者以后，常常参加公益活动。

The difference is mainly in emphasis:

• starting with 自从 highlights the time relationship first
• starting with 她 highlights the person first

Chinese is often flexible in this kind of word order, as long as the sentence remains clear.

The comma separates the time/background clause from the main clause.

Structure:

• 自从她做志愿者以后 = background / time frame
• 常常参加公益活动 = main statement

This is similar to English punctuation in a sentence like:

• Ever since she became a volunteer, she often takes part in charity activities.

Chinese commas are used quite often to divide sentence parts clearly, especially when a sentence begins with a long time phrase or background phrase.