Learn Hardcore Chinese: zǎoshang tā chángcháng cóng dìtiězhàn zǒu dào gōngjiāozhàn zuò gōngjiāochē qù gōngsī. - In the morning she often walks from the subway station to the bus stop and takes a bus to the company.

早上她常常从地铁站走到公交站…
她早上常常从地铁站走到公交站…
她常常早上从地铁站走到公交站… (less common, but possible if you want to stress “in the morning” as the time when this often happens)

Positions that are awkward or wrong:

✗ 她从地铁站常常走到公交站… (splits 从地铁站 unnaturally)
✗ 常常她早上从地铁站走到公交站… (sounds very marked / unusual)

So the given position (after the subject, before the rest of the verb phrase) is the most neutral.

What exactly do 从 and 到 do here? Could I leave one of them out?

从…到… is a very common pattern meaning from … to … in a spatial or temporal sense.

In your sentence:

从地铁站 = from the subway station
到公交站 = to the bus stop

Together:

从地铁站走到公交站 = walk from the subway station to the bus stop.

About omitting them:

If you remove 从, you get 她常常地铁站走到公交站, which is ungrammatical.
If you remove 到, 她常常从地铁站走公交站 is also ungrammatical.

You can sometimes omit 到 if you use 走到那儿, 走过去, etc., but then you usually replace the object with a directional complement. For example:

她常常从地铁站走到公交站。 (full from–to)
她常常从地铁站走过去。 (from the station, walks over there)

In this sentence, you really need both 从 and 到.

Why is it 走到公交站 and not just 走公交站? What does 到 add to 走?

走到 is verb + result complement:

走 = to walk
到 = to arrive, to reach

So 走到公交站 means to walk (until you) reach the bus stop, emphasizing that she arrives there.

If you only say 走 with a place:

在地铁站走 = walk around at the subway station
往公交站走 = walk toward the bus stop (direction, not necessarily arriving)

But ✗走公交站 by itself is not grammatical.

So:

走到公交站 = walk to the bus stop (reach it)
走到家 = walk home (and get there)
走回家 = walk back home

That 到 (or other direction/result words like 到, 回, 进, 出) is very common after movement verbs like 走.

Why don’t we have 在 before 地铁站 or 公交站 (like 在地铁站)? Is 从在地铁站走到在公交站 possible?

Here the focus is on movement from one place to another, so 从 and 到 already serve as the prepositions. You do not add 在 in front of those nouns:

从地铁站走到公交站 ✅
从在地铁站走到在公交站 ❌ (ungrammatical)

We do use 在 when talking about a static location:

她在地铁站等朋友。 = She is at the subway station waiting for a friend.
他在公交站看手机。 = He is at the bus stop looking at his phone.

But when you have a clear movement pattern 从 A 到 B, 在 is normally not used with those nouns.

How can there be so many verbs in a row (走到…坐…去…) with only one subject? Is a connector like 然后 missing?

Chinese often uses serial verb constructions: several actions follow each other in sequence, sharing the same subject, without extra conjunctions.

Your sentence:

…从地铁站　走到公交站　坐公交车　去公司。

Literally:

From the subway station, (she) walks to the bus stop, rides the bus, goes to the company.

All these verbs share the same subject 她, and Chinese is comfortable just listing them in order. You can add connectors if you want to be more explicit:

早上她常常从地铁站走到公交站，然后坐公交车去公司。
早上她常常从地铁站走到公交站，再坐公交车去公司。

然后 / 再 make the sequence extra clear, but they are optional. The original sentence is perfectly natural without them.

Why do we say 坐公交车去公司 and not just 坐公交车公司? What does 去 do here?

Here is the basic pattern:

坐 + (type of transport) + 去 + (place)
= take (transport) to (place)

坐公交车去公司 = take the bus to the company
坐地铁去学校 = take the subway to school
坐飞机去北京 = fly (take a plane) to Beijing

The verb 坐 (“to sit; to ride”) by itself does not take a destination:

✗她坐公交车公司。 (ungrammatical)
她坐公交车去公司。 ✅

In casual speech, 去 can sometimes be dropped if the destination is obvious, but the full, clear structure is 坐…去…. For learners, it’s safer to always include 去 when you mean to (a place).

Why is there no measure word with 公交车? Why not 坐一辆公交车去公司?

Measure words (classifiers) are needed when you:

count things: 一辆公交车 (one bus)
specify a particular one: 那辆公交车 (that bus)

But when the noun is used as a means of transport, you usually don’t put a measure word:

坐公交车去公司。 = take the bus to the company
坐地铁回家。 = take the subway home
坐飞机出国。 = go abroad by plane
坐船去那个岛。 = take a boat to that island

If you want to emphasize one specific bus, you can use a measure word:

她坐一辆公交车去公司。 = She takes one (a) bus to go to the company.

Both are grammatical, but the neutral “by bus” meaning doesn’t need a measure word.

There is no past tense marker like 了. How do we know this means “she often walks…” and not “she walked…” or “she will often walk…”?

Chinese does not mark tense the way English does (past, present, future). Instead it relies on:

Time words: here, 早上 and the habitual adverb 常常.
Context, including surrounding sentences.

In your sentence:

常常 signals a habitual action.
The structure with no 了 normally describes a general fact / routine.

So it naturally corresponds to “In the morning she often walks…” (a regular habit).

Could it refer to the past or future? Yes, in a different context:

Speaking about her old job: the same Chinese sentence, in context, could mean “Back then, in the mornings she often walked…”
Speaking about a plan for her new routine: it could mean “From now on, in the mornings she will often walk…”

Chinese leaves tense more to context, while aspect markers like 了, 过, 着 are used to add other nuances (completion, experience, ongoing state).

Why is it just 去公司 and not 去公司里 or 到公司里? Are those different?

All of these are possible but have slightly different focuses:

去公司
Very common and neutral. Means “go to the company / go to work (at the company).” The idea of entering the physical building is usually implied, but not emphasized.
到公司
Focuses a bit more on the arrival: “get to the company.”
Example: 她八点到公司。 = She arrives at the company at 8.
去公司里 / 到公司里
Add 里 = inside, emphasizing the interior of the company building. These are used more when the “inside” is important:
- 去公司里开会。 = go inside the company (building) to have a meeting.
- 他已经到公司里了。 = he has already gone into the company (he is inside now).

In your sentence, 去公司 is the most natural, neutral choice for “go to the office/company.”