Learn Hardcore Japanese: kinou ha kaze wo hikimasita ga, ima ha daizyoubu desu. - I caught a cold yesterday, but I am okay now.

昨日は literally means "As for yesterday," or "Yesterday (at least / in contrast)".
The rest of the sentence, 風邪を引きましたが、今は大丈夫です, is the comment about yesterday.

So the structure is:

昨日は – as for yesterday
風邪を引きましたが、今は大丈夫です – I caught a cold, but now I'm okay.

You could say the sentence without は:

昨日風邪を引きましたが、今は大丈夫です。

This is also correct, but 昨日は adds a slight feeling of "speaking specifically about yesterday (as opposed to other days)". That contrastive nuance is common with は.

Why don't we say 昨日に here? When do you use に with time words like this?

In Japanese, not all time expressions take に.

Time words that usually do not take に:

Relative days: 今日 (today), 昨日 (yesterday), 明日 (tomorrow)
Regular/frequency words: 毎日 (every day), 毎週 (every week), 来週 (next week)

So you say:

昨日風邪を引きました。 – I caught a cold yesterday.
(Not ✕ 昨日に風邪を引きました in standard Japanese.)

You do use に with:

Specific clock times: ３時に (at 3 o’clock)
Specific dates: １０日に (on the 10th), ４月に (in April)
Some more general times: 朝に (in the morning) – although 朝 is often used without に too.

So:

３時に行きました。 – I went at 3 o’clock.
昨日行きました。 – I went yesterday. (no に)

What is the basic structure of this sentence? How does the Japanese word order map to English?

The sentence is:

昨日は　風邪を引きました　が、　いまは　大丈夫です。

A rough literal breakdown:

昨日は – as for yesterday
風邪を引きました – (I) caught a cold
が、 – but,
いまは – as for now / but now
大丈夫です – (I) am okay

Japanese is topic–comment and verb-final, so the verb comes at the end of the clause:

English: I caught a cold yesterday, but I’m okay now.
Japanese order: Yesterday (topic) cold (object) caught but, now (topic) okay is.

Two key differences for learners:

The topic with は comes first (昨日は, いまは).
The main verb of each clause (引きました, です) comes at the end.

What does 風邪を引きました literally mean? Why do we use を and the verb 引く ("to pull") for catching a cold?

風邪を引きました (かぜをひきました) is the standard way to say "caught a cold".

風邪 (かぜ) – a cold (illness)
を – marks 風邪 as the direct object
引く (ひく) – literally "to pull", but in this expression it means "to catch (a cold)"

So literally, it's something like "pulled a cold", but in natural English we say "caught a cold". It’s an idiomatic expression, just like English "catch a cold" is idiomatic (you’re not literally catching it with your hands).

Important points:

You say 風邪を引く, not ✕ 風邪が引く.
You don’t normally say ✕ 風邪になる for "get a cold" in everyday speech; 風邪を引く is the natural phrase.

Other forms:

風邪を引きます – I catch a cold / will catch a cold (polite, non-past)
風邪を引いています – I have a cold (I’m currently suffering from it)
風邪を引いちゃいました – I ended up catching a cold (casual).

Since we already have 昨日 ("yesterday"), do we still need to use the past tense 引きました? Couldn't it be 引きます?

You still need the past tense.

昨日 tells you when something happened.
ました / ます tell you whether it is past or non-past.

So:

昨日風邪を引きました。 – I caught a cold yesterday. ✅
✕ 昨日風邪を引きます。 – "Yesterday I catch a cold." (ungrammatical / strange)

Time expressions like 昨日 don’t replace verb tense; they just add information. You must still choose the right tense on the verb:

昨日行きました。 – I went yesterday.
明日行きます。 – I will go tomorrow.
いま行きます。 – I’m going now / I’ll go now.

In 引きましたが, what is this が doing? Is it the same が that marks the subject?

Here, が is not the subject marker. It is a conjunction meaning "but / although".

風邪を引きました – I caught a cold
が、 – but,
いまは大丈夫です – (I) am okay now

So 引きましたが = "(I) caught a cold, but..."

There are two main uses of が:

Subject marker が
- 犬が好きです。 – I like dogs. (dogs = subject)
Conjunction が (as in this sentence)
- 忙しいですが、行きます。 – I’m busy, but I’ll go.

You can often replace this conjunction が with:

けど (more casual):
- 風邪を引いたけど、いまは大丈夫です。
でも (starts a new sentence):
- 風邪を引きました。でも、いまは大丈夫です。 – I caught a cold. But I’m okay now.

The sentence doesn't say "I". How do we know the subject, and how would we say it if we wanted to be explicit?

Japanese often drops the subject (and even the topic) when it’s obvious from context.

In this sentence, the default assumption is that the speaker is talking about themselves. So:

昨日は風邪を引きましたが、いまは大丈夫です。
→ "I caught a cold yesterday, but I am okay now."

If you want to make it explicit, you can add 私 ("I"):

私は昨日風邪を引きましたが、いまは大丈夫です。

This is perfectly correct, but in everyday conversation Japanese speakers often skip 私は unless:

they need to contrast subjects:
- 私は大丈夫ですが、弟はまだ熱があります。
  – I am okay, but my younger brother still has a fever.
or clarify who you’re talking about if it’s ambiguous.

So, the "default subject" in many personal statements is the speaker, unless the context clearly suggests someone else.

Why do we say いまは (今は) with は again? What's the difference between 今は大丈夫です and 今大丈夫です?

今は uses は as a topic/contrast marker, similar to 昨日は.

今は大丈夫です。 – As for now, I’m okay.
今大丈夫です。 – I’m okay now / I’m okay right now.

Nuance:

今は大丈夫です often implies a contrast with another time, e.g.
"I wasn’t okay before, but now (at least) I’m okay."
It can subtly suggest "for the time being" or "these days".
今大丈夫です is more neutral, just stating your current state:
e.g. someone asks if you’re free now:
- いま大丈夫ですか。 – Are you free now?
- はい、いま大丈夫です。 – Yes, I’m free now.

In this sentence, 今は大丈夫です nicely contrasts:

昨日は (yesterday: bad condition)
今は (now: okay condition)

What exactly does 大丈夫 mean here? Is it "I'm healthy", "I'm safe", or just "it's okay"? How is it used grammatically?

大丈夫 (だいじょうぶ) is quite flexible. Here it means roughly "okay / all right / fine".

In this context:

いまは大丈夫です。
→ "I’m okay now." / "I’m all right now."
(Implying you’re no longer seriously suffering from the cold.)

Grammatically:

大丈夫 is a na-adjective (形容動詞).
- Dictionary form: 大丈夫だ
- Polite: 大丈夫です

Examples:

この薬は大丈夫です。 – This medicine is okay (safe / acceptable).
一人で大丈夫です。 – I’ll be fine on my own. / I can manage alone.
大丈夫？ – Are you okay? (very casual)

Compared with 元気 (げんき):

元気です。 – I’m well / I’m lively / I’m in good spirits.
大丈夫です。 – I’m okay / It’s not a problem / I’m all right.

After a cold, both are possible but slightly different:

もう元気です。 – I’ve recovered; I feel energetic again.
もう大丈夫です。 – I’m okay now; I’m no longer in trouble from the cold.

What level of politeness is 風邪を引きましたが、いまは大丈夫です? How would this change in casual conversation?

This sentence uses the polite style:

引きました – polite past (ます form)
大丈夫です – polite です

So it’s appropriate for:

talking to people you don’t know well
coworkers, teachers, customers, etc.
most everyday polite conversation.

A casual version (to friends, family, etc.) might be:

昨日風邪引いたけど、いまは大丈夫。
(Plain past 引いた, drop です.)

Even more colloquial pronunciation/spelling:

昨日風邪ひいたけど、今はもう大丈夫。

Changes you see in casual speech:

ました → た
- 引きました → 引いた
dropping です / だ at the end when clear from context
using けど instead of が more often in speech.