If and if only: अगर … तो, काश

For things that may be true or may still happen, use अगर … तो … (agar … to …), if … then …, with normal tenses. तो (to) starts the second part and is usually kept.

The अगर part can be in the present, the future, the subjunctive, or even the simple past for something that may happen later. The तो part is usually a future or a request.

For things that are not true, or did not happen, both parts take the stem + -ता / -ती / -ते / -तीं (-taa / -tee / -te / -teen), without है or था. It agrees like an adjective. “Not” is usually न (na) or नहीं (naheen).

The same sentence can be about now or about the past: अगर मेरे पास पैसे होते, तो मैं एक नया घर ख़रीदती (agar mere paas paise hote, to main ek nayaa ghar khareedtee) means “if I had the money, I'd buy a new house” or “if I had had the money, I would have bought a new house” (a woman speaking). The situation makes it clear.

To make it clear that you mean a finished action in the past, put होता (hotaa) after the simple past: अगर हम जल्दी निकल गए होते, तो बस नहीं छूटती (agar ham jaldee nikal gaye hote, to bas naheen chhoottee), if we had left earlier, we wouldn't have missed the bus. With ने (ne) verbs, the verb and होता both agree with the object, or stay masculine if there is none: अगर तुमने मुझे बताया होता, … (agar tumne mujhe bataayaa hotaa, …), if you had told me, …; अगर मैंने वह फ़िल्म देखी होती, … (agar mainne vah film dekhee hotee, …), if I had seen that film, …

काश (kaash), if only, I wish, is followed by the same -ता (-taa) form: काश दिल्ली इतनी दूर न होती! (kaash Dillee itnee door na hotee!), if only Delhi weren't so far away! काश मेरे पास ज़्यादा समय होता! (kaash mere paas zyaadaa samay hotaa!), if only I had more time!

Two more unreal forms make the time clear. For an unreal action in progress, use the continuous with होता (hotaa): अगर हम पहाड़ों में काम कर रहे होते, तो साफ़ हवा में साँस ले रहे होते (agar ham pahaaron men kaam kar rahe hote, to saaf havaa men saans le rahe hote), if we were working in the hills now, we'd be breathing clean air.

For a finished past action that didn't happen, use the simple past + होता: अगर हमने पहले ध्यान दिया होता, तो उनकी हालत इतनी ख़राब नहीं होती (agar hamne pahle dhyaan diyaa hotaa, to unkee haalat itnee kharaab naheen hotee), if we had paid attention earlier, she wouldn't have got so ill. The result part can take the same form or, as here, just the -ता (-taa) form. To say what may have happened, use the simple past + हो (ho): हो सकता है कि उसने देखा हो (ho saktaa hai ki usne dekhaa ho), she may have seen it.