Examples
Meaning
を marks the direct object of a verb. It shows what the action is done to, such as the food you eat or the book you read.
Formation
Put を after the noun that receives the action. The particle を is written with the kana を, but it is usually pronounced o in modern Japanese.
Usage
Use を with action verbs such as 食べます, 飲みます, 読みます, 書きます, 見ます, and 勉強します. The noun before を is the target of that action.
Nuance
を does not make the sentence polite or casual. It simply tells the listener which thing the action is directed at. In beginner Japanese, it is one of the clearest ways to connect a noun and an action verb.
Comparison
は and が mark topic or subject, but を marks the object. も can replace を when you want to say that the object is also included, as in 水も飲みます.
Common Mistakes
A common mistake is reading を as wo in normal sentences. Another mistake is using が for the object of action verbs, such as 本が読みます, when the intended meaning is 本を読みます.
Related Grammar
は vs が
は (wa) marks the topic of a sentence, while が (ga) marks the subject or focused new information. This page explains the basic N5 difference with simple examples.
も
も (mo) means also or too. It replaces particles such as は, が, or を when adding another person or thing to the same statement.
で
で (de) marks where an action happens or what means, tool, or language is used. It is different from に, which often marks existence or destination.