Learn Hardcore Japanese: sigoto ga isogasii to, ziyuuna zikan ga herimasu.

The particle が marks the subject of the adjective 忙しい. In Japanese, when you describe something with an i-adjective, you use が to indicate what is “busy.” Here it literally means “work is busy.”

Why is 忙しい in the plain form before と, and what does と mean here?

The conditional particle と attaches to the plain non-past (dictionary) form of verbs and adjectives. In this use, と means “whenever” or “when.” So 仕事が忙しいと conveys “whenever work is busy…”

Why does と attach directly to 忙しい without any extra particle?

Plain-form adjectives in Japanese can take と directly to form a conditional. There’s no need for an additional particle between the adjective and と—you simply say 忙しいと.

What kind of adjective is 自由 and why do we write 自由な時間?

自由 is a na-adjective. When a na-adjective modifies a noun, it requires な before the noun. Hence 自由な時間 means “free time.”

Why is 時間 marked with が in 時間が減ります, not を?

The verb 減る is intransitive (“to decrease”). Intransitive verbs take が to mark the thing undergoing the change. So 時間が減ります literally means “time decreases.”

What’s the difference between 減る and 減らす?

減る (intransitive): something decreases on its own (時間が減る = time decreases).
減らす (transitive): someone or something causes a decrease (時間を減らす = to reduce time).

Could you use another verb like 少なくなる instead of 減る? How would that change the nuance?

Yes. You can say 自由な時間が少なくなります (“free time becomes fewer”).

減る focuses on the decrease itself.
少なくなる emphasizes shifting into a state of having “not much.”
In this sentence they’re largely interchangeable.

How does the と conditional differ from たら or なら?

と expresses a general or automatic result: “whenever/as soon as …, then ….”
たら is for specific or hypothetical situations: “once/if … happens, then ….”
なら sets the first clause as context or assumption: “if it’s the case that …, then ….”
Here と highlights an inevitable pattern: whenever work is busy, free time inevitably decreases.