Learn Hardcore Japanese: atarasii sigoto ga hazimatte kara, huan ga sukosizutu hette kimasita. - Since my new job started, my anxiety has gradually decreased.

新しい仕事は始まってから… – possible, but now 新しい仕事 is a topic: “As for my new job, after it started, …” This sounds like you are going to say different things about it (maybe the job is hard, but as for my anxiety… etc.)
不安は少しずつ減ってきました – also possible, but it sounds like “As for my anxiety, (it) has gradually decreased,” often implying contrast (e.g., the stress is still there, but the anxiety has gone down).

In a neutral, matter-of-fact statement with no contrast, が is the most natural choice for both nouns here.

What does 始まってから mean, and why is it 始まってから, not 始まったから?

The pattern here is:

V‑て + から → “after doing V” / “since doing V” (time sequence).

So:

始まってから literally: “after (it) has started / since (it) started”.

If you say:

始まったから → this is plain past + から, and usually means “because it started” (expressing a reason, not a time sequence).

So:

新しい仕事が始まってから、不安が…
→ “After/since my new job started, my anxiety…”
新しい仕事が始まったから、不安が…
→ “Because my new job started, my anxiety…” (emphasizing cause; sounds more like you are blaming the new job).

The sentence you gave is about when the change in anxiety started, so 始まってから (time “after/since”) is correct and natural.

Is から here “since” or “because”? How can I tell?

Here, から is used in a time sense: “since / after”.

Clues:

It follows a verb in て-form: 始まってから → typical pattern for “after doing…”.
The English meaning is “Since my new job started, my anxiety has gradually decreased” — that “since” is a starting point in time, not a reason like “because”.

General guideline:

V‑てから → almost always “after V / since V” (time).
V‑(plain) から (especially when giving an explanation) → often “because V” (reason).

So here, because it’s 始まってから (V‑てから) and it introduces a time period, you should read it as “since/after,” not “because.”

What is literally happening in 減ってきました? What does きました add?

Breakdown:

減る – “to decrease”
減って – て-form
減ってくる – “to come to decrease,” i.e., “to start to decrease and move toward the present”
減ってきました – polite past of 減ってくる

〜てくる (V-て + くる) often expresses:

A change that began in the past and has been developing up to now.
Something that has come about or has started to happen and reached the present.

So 不安が少しずつ減ってきました implies:

“My anxiety has been gradually decreasing (up to now).”
It feels like a process over time that has led to the current situation, not just a single drop at one moment.

What’s the difference between 減ってきました and just 減りました?

Both involve “decreasing,” but the nuance differs:

減りました
- Simple past / completed action.
- “(It) decreased” / “(It) has decreased.”
- Focuses on the result (it is now less), not how it happened over time.
減ってきました
- Uses 〜てくる, emphasizing a process leading up to now.
- Suggests it gradually decreased over some span of time and this change has reached the present.
- Very close to English “has been decreasing” or “has gradually come down.”

In your sentence, 少しずつ already means “little by little,” and 減ってきました matches that idea very naturally: a gradual change over time.

Could I say 減っていきました instead? What’s the difference between 〜てくる and 〜ていく?

〜てくる and 〜ていく both attach to the て-form of a verb, but their perspectives differ:

〜てくる
- Change approaches the present or current viewpoint.
- Often: “has come to V / has started to V and now is like that.”
- Used for changes that lead up to now.
〜ていく
- Change goes away from the present into the future.
- Often: “will continue to V / will go on V-ing from now.”
- Used for changes that will continue from this point onward or that moved away in time/space.

So:

不安が少しずつ減ってきました。
→ “My anxiety has been gradually decreasing (up to now).”
(Looking back at the change up to the present.)
不安が少しずつ減っていきました。
→ “My anxiety gradually went on decreasing.”
(More like narrating a past sequence: from some point, it kept decreasing over time, moving away from that reference point.)

In many contexts, both could be understandable, but 減ってきました fits best when you are talking about your current mental state and how it has changed up to now.

Why is it past tense きました instead of きています or きます?

Nuance of each:

減ってきました
- Describes a change that has been happening and has reached the present.
- Very similar to English present perfect: “has been decreasing / has come to decrease.”
- Appropriate when you’re summarizing the situation up to now.
減ってきています
- Emphasizes the on-going process right now more strongly:
  “(It) is in the process of decreasing (and has been).”
- Grammatically OK, but sounds a bit heavier / more technical here. Daily speech usually sticks with 減ってきました.
減ってきます
- Non-past; usually indicates future or general behavior:
  - “(It) will decrease (over time).”
  - “(It) tends to decrease (over time).”
- Would not match 始まってから (which refers to a period from a point in the past up to now).

So 減ってきました is chosen because you are talking about a change that has already been happening since the job started and has led to your current state.

What does 少しずつ mean exactly, and how is it different from 少し or だんだん?

少しずつ
- Literally: “little by little.”
- Emphasizes many small changes or steps accumulating over time.
- Perfect match with things that gradually change: 少しずつ慣れる (get used to something little by little).
少し
- “A little / a bit.”
- Can describe:
  - a small amount: “a little (quantity)”
  - a small degree: “slightly”
- Not necessarily repeated steps.
  e.g. 不安が少し減りました = “My anxiety decreased a little (by a small amount).”
だんだん
- “Gradually / step by step / more and more.”
- Focuses on the progressive change itself, not specifically on “small increments.”
- e.g. だんだん寒くなってきました = “It’s gradually getting colder.”

In your sentence, 少しずつ stresses that your anxiety has gone down in small steps over time, not in one big drop.

Where can I put 少しずつ in the sentence? Does its position matter?

少しずつ is an adverb, and adverbs in Japanese are fairly flexible. Natural positions include:

The original:
- 不安が少しずつ減ってきました。
Or:
- 少しずつ不安が減ってきました。

Both are common. Nuance difference here is very small:

不安が少しずつ減ってきました。
– Slightly more focus on 不安 first (“My anxiety has… little by little decreased”).
少しずつ不安が減ってきました。
– Slightly stronger emphasis that what happened was “little by little”.

Unnatural:

不安が減ってきました少しずつ。 – sounds wrong; adverbs like 少しずつ should not be stuck on the very end like that.

In most cases, putting 少しずつ right before the verb phrase (減ってきました) or right before the subject phrase (不安が) is fine and natural.

Why is it 新しい仕事が始まってから and not 新しい仕事を始めてから? What’s the difference between 始まる and 始める here?

始まる and 始める are a common verb pair:

始まる – intransitive: “to begin / to start (on its own).”
- Subject is the thing that begins.
- 新しい仕事が始まる – “The new job starts.”
始める – transitive: “to begin (something).”
- Someone (you, they, etc.) starts an action or thing.
- 新しい仕事を始める – “(I) start a new job.”

So:

新しい仕事が始まってから
→ Focuses on the job itself starting as an event/situation:
“After the new job started…”
新しい仕事を始めてから
→ Focuses on your action of starting the job:
“After (I) started the new job…”

Both are grammatically OK and both are used in real Japanese. The nuance is subtle:

If you’re thinking of the job more as a situation that began (new environment, schedule, etc.), 仕事が始まってから feels very natural.
If you want to emphasize your personal action or decision (“once I started a new job…”), 仕事を始めてから is also perfectly fine.

Could I say 不安は少しずつ減ってきました instead of 不安が? What’s the nuance?

Yes, you can say:

不安は少しずつ減ってきました。

But the nuance changes:

不安が少しずつ減ってきました。
- Neutral statement: “My anxiety has gradually decreased.”
- 不安 is the grammatical subject.
不安は少しずつ減ってきました。
- 不安 becomes the topic.
- Implies something like “As for my anxiety, it has gradually decreased…”
- Often hints at contrast:
  “As for my anxiety, it’s decreased (but something else hasn’t),” or
  “My anxiety has decreased (though other problems remain).”

If you’re just straightforwardly reporting what happened with no contrast, 不安が is more neutral and typical. 不安は is used when the anxiety is already established in the conversation as a topic, or when you want that contrasting feel.

How would this sentence look in casual speech instead of the polite 〜ました form?

The casual version simply uses the plain past instead of 〜ました:

新しい仕事が始まってから、不安が少しずつ減ってきた。

Differences:

減ってきました → polite
減ってきた → casual/plain

Everything else (particles, word order, etc.) can stay the same. Among close friends or in an informal diary, you’d naturally use 減ってきた. In more formal situations or when talking to people you should be polite to, 減ってきました is appropriate.