क्योंकि (kyonki)

conjunction A single-sense word: the everyday conjunction meaning 'because', used to introduce a reason clause.
Sense 1 : because
conjunction Introduces a subordinate clause that gives the reason for the main clause; the main clause usually comes first, e.g. main clause + क्योंकि (kyonki) + reason clause.
Synonyms: चूँकि (choonki, literary, precedes the reason clause and pairs with इसलिए (islie))
Frequency: high
Cultural Note: This is simply the everyday word for 'because' that you will hear constantly in explanations, excuses, and arguments alike -- from a child explaining to a parent why homework isn't done, to a friend explaining why they are late. It is one of the first conjunctions learners should make fully active, since giving reasons comes up in almost every conversation.
Common Mistakes: 1) Learners sometimes try to open a sentence with क्योंकि (kyonki) the way English can start with 'because', but the natural Hindi order puts the result first: मैं नहीं आया क्योंकि बारिश हो रही थी (main nahiin aayaa kyonki baarish ho rahii thii, 'I didn't come because it was raining') rather than leading with the reason. 2) Do not pair क्योंकि (kyonki) with इसलिए (islie) in the same sentence -- unlike चूँकि (choonki), which forms a correlative pair with इसलिए (islie), क्योंकि (kyonki) stands alone.
Sample Sentences:
मैं आज नहीं आ सकता क्योंकि मुझे बुखार है। I can't come today because I have a fever.
(main aaj nahiin aa saktaa kyonki mujhe bukhaar hai।)
Register: Casual -- Mood: Indicative -- Tense: Present -- Voice: Active
वह स्कूल नहीं गई क्योंकि उसकी तबीयत ठीक नहीं थी। She didn't go to school because she wasn't feeling well.
(vah skool nahiin gaii kyonki uskii tabiiyat thiik nahiin thii।)
Register: Neutral -- Mood: Indicative -- Aspect: Perfective -- Tense: Past -- Voice: Active
मुझे देर हो गई क्योंकि रास्ते में बहुत ट्रैफ़िक था। I got late because there was a lot of traffic on the way.
(mujhe der ho gaii kyonki raaste men bahut traifik thaa।)
Register: Casual -- Mood: Indicative -- Aspect: Perfective -- Tense: Past -- Voice: Active
जल्दी सो जाओ क्योंकि कल सुबह जल्दी उठना है। Go to sleep early because you have to wake up early tomorrow.
(jaldii so jaao kyonki kal subah jaldii uthnaa hai।)
Register: Casual -- Mood: Imperative -- Voice: Active
अगर वह मुझे बताता कि वह आ रहा है, तो मैं तैयार होता, क्योंकि अभी घर बहुत गंदा है। If he had told me he was coming, I would have been ready, because the house is very messy right now.
(agar vah mujhe bataataa ki vah aa rahaa hai, to main taiyaar hotaa, kyonki abhii ghar bahut gandaa hai।)
Register: Neutral -- Mood: Counterfactual -- Aspect: Habitual -- Voice: Active
शायद वह नहीं आया क्योंकि उसे हमारी बात याद ही नहीं रही होगी। Maybe he didn't come because he probably didn't even remember what we said.
(shaayad vah nahiin aayaa kyonki use hamaarii baat yaad hii nahiin rahii hogii।)
Register: Casual -- Mood: Subjunctive -- Aspect: Perfective -- Tense: Past -- Voice: Active
इस योजना को रद्द कर दिया गया क्योंकि पर्याप्त धन उपलब्ध नहीं था। This plan was cancelled because sufficient funds were not available.
(is yojnaa ko radd kar diyaa gayaa kyonki paryaapt dhan uplabdh nahiin thaa।)
Register: Formal -- Mood: Indicative -- Aspect: Perfective -- Tense: Past -- Voice: Passive
आदरणीय सदस्यों को सूचित किया जाता है कि बैठक स्थगित की जा रही है, क्योंकि अध्यक्ष महोदय अस्वस्थ हैं। The respected members are informed that the meeting is being postponed, because the chairman is unwell.
(aadarniiy sadasyon ko soochit kiyaa jaataa hai ki baithak sthagit kii jaa rahii hai, kyonki adhyaksh mahoday asvasth hain।)
Register: Formal -- Mood: Indicative -- Aspect: Continuous -- Tense: Present -- Voice: Passive
मैं हर साल दिवाली पर घर जाता हूँ क्योंकि परिवार के साथ त्योहार मनाना ज़रूरी लगता है। I go home every year on Diwali because celebrating the festival with family feels important.
(main har saal divaalii par ghar jaataa hoon kyonki parivaar ke saath tyohaar manaanaa zaroorii lagtaa hai।)
Register: Neutral -- Mood: Indicative -- Aspect: Habitual -- Tense: Present -- Voice: Active
वह गुस्से में चली गई क्योंकि किसी ने भी उसकी बात नहीं सुनी। She left angrily because no one listened to what she had to say.
(vah gusse men chalii gaii kyonki kisii ne bhii uskii baat nahiin sunii।)
Register: Casual -- Mood: Indicative -- Aspect: Perfective -- Tense: Past -- Voice: Active
हमें और मेहनत करनी होगी क्योंकि प्रतियोगिता बहुत कठिन होगी। We will have to work harder because the competition is going to be very tough.
(hamen aur mehnat karnii hogii kyonki pratiyogitaa bahut kathin hogii।)
Register: Neutral -- Mood: Indicative -- Tense: Future -- Voice: Active