Learn Hardcore Japanese: kare ha mendouna koto wo ato ni surukoto ga ooi ga, hiruma ni youzi wo sumaseru you ni suru. - He often puts troublesome things off, but he tries to get errands done during the day.

あとにする means “to leave something for later; to postpone it.” It’s common in everyday speech and can also appear in the request form それはあとにして (“let’s do that later” / “not now, please”).
後回しにする also means “to put off; to push down the schedule,” and is a bit more explicit/formal about deferring priority.
In this sentence, 面倒なことをあとにする ≈ “(he) leaves troublesome things for later.” You could swap in 後回しにする with almost no change in meaning, just a slightly stronger “procrastination” feel.

Why are there two instances of こと in 面倒なことをあとにすることが多い?

They’re different “こと”s:

First こと in 面倒なこと = “things” (nominal part of the adjective phrase “troublesome things”).
Second こと nominalizes the verb phrase あとにする (“to put off”), turning it into a noun-like chunk: あとにすること = “the act of putting off.”
Put together: 面倒なことをあとにすることが多い → “The act of putting troublesome things off is frequent (for him).”
A smoother alternative that avoids the double こと is 面倒なことを後回しにしがちだ (“he tends to put off troublesome things”).

What does 〜することが多い express? How is it different from 〜することがある?

〜することが多い = “often does; tends to do.” It indicates high frequency or a strong tendency.
〜することがある = “there are times when …; sometimes.” It indicates occasional occurrence without implying frequency.

Is the が after 多い the subject marker? What is it doing here?

Here が is the conjunction “but/although,” linking the first clause to the second: “He often puts off troublesome things, but ….” It is not the subject-marking が. You could also use けど/けれど(も); が sounds a bit more formal/written.

How does 〜ようにする work, and how is it different from 〜ようとする and 〜ようにしている?

〜ようにする: “make an effort to do; try to make it a practice.” It’s about adopting a policy/effort, not a one-off attempt.
〜ようとする: “try/attempt (right then).” Often describes an immediate attempt and can imply it didn’t succeed.
〜ようにしている: “I make it a rule to …; I keep doing … as a habit.” It emphasizes an established routine over time. Example: 早く寝るようにする (I’ll try to go to bed early) vs 早く寝ようとする (I’m trying to go to bed early right now) vs 早く寝るようにしている (I make it a habit to go to bed early).

Why 昼間に and not 昼に or 日中に? Are they different?

昼間 = “daytime (as opposed to evening/night).” Good when contrasting with nights.
昼 = “noon / around midday.” It can mean the general daytime in casual talk, but it often sounds like the midday period.
日中 = “during the day (while the sun is up),” a bit more neutral/formal. In this sentence, 昼間に emphasizes doing errands in the daytime rather than at night. 昼に narrows the window to midday-ish. 日中に would also work and sounds a touch more formal. You could also say 昼間のうちに (“by the end of the daytime”).

What exactly does 用事 mean?

用事 is “errands; things to take care of; personal business.” Common collocations:

用事がある = “I have something to do/an errand.”
用事を済ませる = “to get one’s errands done; to take care of business.” It’s different from:
仕事 (work/job duties),
宿題 (homework),
予定 (a scheduled plan/appointment).

What’s the difference between 済ませる, 済む, and words like 終える/終わらせる/片付ける?

済む (intransitive): “to be finished/settled/suffice.” Example: それで済む (“that will do/that’s enough”).
済ませる (transitive): “to finish/settle (something).” Example: 用事を済ませる (take care of one’s errands).
済ます: a slightly more casual variant of 済ませる.
終える/終わらせる: “to finish/bring to an end.” Works with tasks: 仕事を終える.
片付ける: “to put away/clean up/deal with.” With tasks, it implies “tidy up/dispatch it”: 用事を片付ける also sounds natural.

Could I say 面倒なことはあとにすることが多い instead of 面倒なことをあとにすることが多い?

Yes. The nuance changes:

を keeps a neutral object focus: “He often leaves troublesome things for later.”
は topicalizes and adds contrast: “As for troublesome things, he tends to leave them for later (implying other things might be different).”

Who is the subject of the second clause? Why isn’t 彼 repeated?

彼は sets the topic for the whole sentence, so it carries over to the second clause by default. The understood subject remains “he.” Repeating 彼は before the second clause is possible but unnecessary unless you need extra clarity or emphasis.

Is あとにして the same thing as あとにする?

They’re related forms of the same expression:

あとにする is the dictionary form used for statements: それをあとにする (“leave that for later”).
あとにして is the -te form, often used as a request/command: それ、あとにして (“do that later” / “not now”).

If I want to emphasize a steady habit rather than just “trying,” should I change anything?

Yes—use 〜ようにしている. For example:

彼は面倒なことを後回しにしがちだが、昼間に用事を済ませるようにしている。 This indicates he has made it his ongoing habit/rule to finish errands during the day.

Is the contrast with が here natural? “He puts off troublesome things, but he tries to finish errands during the day.”

Yes. 面倒なこと is a broad category of bothersome tasks; 用事 often refers to time-bound errands or personal business (e.g., going to the bank/post office). The contrast suggests: he tends to procrastinate on bothersome stuff in general, but at least he makes an effort to get daytime errands done.

Could I use あとでやる instead of あとにする?

You can in many contexts, but the grammar and feel differ:

あとでやる = “do it later” (focus on doing later).
あとにする = “leave it for later” (focus on postponing now). With a direct object, 〜をあとにする is a neat, compact way to say “postpone (X).” 〜をあとでやる is fine but a bit more casual and explicit about the “doing.”

Can I rephrase the second half as 用事は昼間に済ませるようにする? Any nuance change?

Yes. Topicalizing 用事 with は shifts emphasis to “as for errands, (he) tries to do them in the daytime,” which is useful if you’re contrasting with, say, studying or hobbies at night. The original 昼間に用事を… is a more neutral statement of time + object.