の and こと: treating a clause like a noun
Use の or こと after a verb clause in the plain form to nominalize an action or fact. Compare their tendencies with preferences, definitions, and perception, while noting structures where they are not freely interchangeable.
Create a place for a particle after a clause
毎朝少し歩くのは気持ちがいいです。 uses の to turn the clause 毎朝少し歩く into a noun-like unit that can take は. The whole activity becomes the topic of the sentence.
新しい表現を使うことに慣れました。 uses こと to nominalize the preceding clause and connect it to に. Depending on its role, a nominalized clause can be followed by particles such as は, が, を, or に.
Choose a form for preferences and definitions
妹は写真を撮るのが好きです。 uses の naturally for a familiar, concrete activity that the speaker's sister likes. This tendency does not make こと grammatically impossible in every preference statement.
私の目標は日本語で日記を書くことです。 defines writing a diary in Japanese as the speaker's goal. こと is common when an activity is treated more abstractly, as in a goal, rule, or general statement.
Check perception verbs and fixed structures
窓から、子どもが帰ってくるのを見ました。 says that, from the window, the speaker saw the concrete event of a child coming home. のを is natural before perception verbs such as 見る and 聞く in this use.
ここでは写真を撮ることができます。 contains the established ability construction ことができる. の and こと overlap in some contexts, but do not assume that they can be exchanged in every structure.
Quick check: To say that you saw a bird land in a tree, is 鳥が木に止まるのを見ました more natural than 鳥が木に止まることを見ました?
Show the answer and why
Yes. のを naturally presents the concrete event as the object of 見ました.