って（主題）

って is a versatile particle in spoken Japanese, often functioning as a casual equivalent of は, or a contraction of とは / というのは. Its primary functions include:

• Spoken Topic Marking: It functions like 'Speaking of...' or 'As for...'. It has a strong 'topic-initiating' feel, making it highly preferred when suddenly bringing up a new topic. While it is perfectly natural for describing subjective evaluations with adjectives (e.g., 「犬って可愛い」 'Dogs are cute'), when used for objective statements with a noun + copula (だ) or for describing past events or actions, it typically requires conversational sentence endings like んだ, だよ, or よね to sound natural and provide interactional modality (e.g., 「山口さんって先生なんだよ」 'Yamaguchi's a teacher, you know', or 「太郎って昨日東京に行ったんだよ」 'Taro went to Tokyo yesterday, you know'). This is because って carries a strong sense of subjectivity, emotional closeness, or personal evaluation toward the topic, making it pair well with opinions and poorly with flat, objective news reporting. Furthermore, while it replaces が and を entirely, it can also replace は in compound particles like までは (までって). Note that では usually becomes じゃ in casual speech, and からって is primarily used for its concessive meaning ('just because/even if'). For には, the に can be dropped entirely for locative or domain topics (e.g., changing 「東京には人が多い」 'There are many people in Tokyo' to 「東京って人が多い」 'Speaking of Tokyo, there are many people'). However, for dative or target topics (e.g., 「田中さんには」 'To Tanaka'), dropping に changes the meaning entirely, so they usually remain には or に in casual speech. Finally, while は is the standard choice for strong contrast, って can still be used in contrastive contexts in casual speech (e.g., 「犬って可愛いけど、猫って冷たいよね」 'Dogs are cute, but cats are cold, aren't they?').
• Defining/Echoing: It is heavily used to bring up a topic that the other person just mentioned (a 'conversational echo'), define a word, or repeat something someone else just said to ask for clarification (e.g., 'What does [Word] mean?'). It is also common to use it with ellipsis, dropping the rest of the sentence to form a question (e.g., 「明日の会議って？」 meaning 'What about tomorrow's meeting?'). This often carries a nuance of surprise or disbelief (e.g., 「明日の会議って！？」 'Wait, tomorrow's meeting!?').
• Distinguishing Functions: Be careful not to confuse this topic marker with its other roles: quotation (replacing と), hearsay (meaning 'I heard', equivalent to だそうだ / と言っていた, often used at the end of a sentence, e.g., 「太郎も行くって」 'Taro said he's going too'), naming/defining (a contraction of という, e.g., 「ポチっていう犬」 'A dog named Pochi'), or the concessive function (たって / だって, meaning 'even if'). Note that when attached to a person's name, it can be ambiguous; depending on context, 「田中さんって…」 could mean 'Speaking of Tanaka...' (topic) or 'I heard it's Tanaka' / 'Someone said "Tanaka"' (quotation/hearsay).
• Register: This is a spoken-style particle. While common in friendly, informal polite (ます form) conversation, it is a casualism and should be avoided in strictly formal, professional, or honorific contexts. In written, academic, or formal contexts, you must revert って back to は, とは, or というのは depending on its specific underlying function in the sentence.

Morphology:
It attaches directly to nouns, and the plain form of verbs and adjectives (attaching directly to a noun includes な-adjective stems). When attached to verbs and adjectives, it acts as a nominalizer by contracting というのは, meaning 'the act of [verb]ing' or 'being [adjective]' (e.g., 「一人で暮らすって寂しい」 = 'Living alone is lonely'). Additionally, speakers frequently insert the nominalizer の before って when attaching it to verbs and adjectives (e.g., 食べるのって難しい 'Eating is difficult'). For nouns and な-adjectives, it attaches directly to the word (e.g., 先生って 'Speaking of the teacher', きれいって 'Speaking of being pretty'). In natural, rapid speech, って often softens to て when the preceding word ends in the mora ん (e.g., 先生て 'Speaking of the teacher'). It can also attach to the plain form だ (e.g., 先生だって 'Speaking of the teacher' / 'Even the teacher') as a contraction of だというのは, continuing to function as a topic marker, though it can also function as a hearsay or concessive marker depending on context. Furthermore, when attached to the た form of a verb (e.g., 行ったって 'Even if I go'), it can sometimes function as a concessive marker meaning 'even if' depending on context. As a topic marker, it is usually followed by a pause or a comment about the preceding phrase. In casual speech, it is also frequently followed by conversational fillers like さ or ね (e.g., 「あの人ってさ…」 'Speaking of that person...') to soften the sentence or hold the floor.