Should and have to: चाहिए, है, पड़ना

मुझे … चाहिए (mujhe … chaahiye) with a noun means “I need”. With a verb in the -ना (-naa) form it means “should”: आपको आराम करना चाहिए (aapko aaraam karnaa chaahiye), you should rest. The person takes को (ko) or its short form: मुझे, तुम्हें, आपको, उसे, हमें, उन्हें (mujhe, tumhen, aapko, use, hamen, unhen).

For “shouldn't”, put नहीं (naheen) before the verb. For “should have”, add था (thaa): मुझे पहले आना चाहिए था (mujhe pahle aanaa chaahiye thaa), I should have come earlier.

The same pattern with है (hai) says what you have to do or are going to do: मुझे कल जल्दी उठना है (mujhe kal jaldee uthnaa hai), I have to get up early tomorrow. It often describes plans and duties.

The negative मुझे नहीं जाना है (mujhe naheen jaanaa hai) often means “I don't want to go”.

With पड़ना (parnaa) the pattern stresses that circumstances force you: आपको पुल से जाना पड़ेगा (aapko pul se jaanaa paregaa), you'll have to take the bridge. पड़ना changes for tense: पड़ता है (partaa hai), usually have to; पड़ा (paraa), had to; पड़ेगा (paregaa), will have to.

In all three patterns, if the verb has an object, the -ना (-naa) form and पड़ना agree with it. With a feminine object, -ना becomes -नी (-nee): आपको दवा लेनी चाहिए (aapko davaa lenee chaahiye), you should take medicine; हमें ट्रेन बदलनी पड़ेगी (hamen tren badalnee paregee), we'll have to change trains. With a masculine plural object it becomes -ने (-ne): मुझे कुछ कपड़े ख़रीदने हैं (mujhe kuchh kapre khareedne hain), I have to buy some clothes.

The verb never agrees with the person: with no object it stays in -ना, so a woman also says मुझे जाना है (mujhe jaanaa hai). With चाहिए (chaahiye), you'll also hear the -ना form with feminine objects in everyday speech.

ज़रूरी (zarooree), important, necessary, gives serious advice. For a general rule use the -ना (-naa) form: रोज़ चलना ज़रूरी है (roz chalnaa zarooree hai), walking every day is essential. For one person use कि (ki) and the subjunctive: यह ज़रूरी है कि आप अपनी आदतें बदलें (yah zarooree hai ki aap apnee aadaten badlen), it's important that you change your habits.

In short: चाहिए (chaahiye) is advice, है (hai) a plan or duty, पड़ना (parnaa) no choice, and ज़रूरी है (zarooree hai) something really necessary.

Put था (thaa) after चाहिए (chaahiye) for regrets: मुझे ज़्यादा मेहनत करनी चाहिए थी (mujhe zyaadaa mehnat karnee chaahiye thee), I should have worked harder. The -ना (-naa) form and था agree with a feminine or plural object: मुझे अपनी ग़लतियाँ देखनी चाहिए थीं (mujhe apnee galtiyaan dekhnee chaahiye theen), I should have looked at my mistakes.

सकता था (saktaa thaa) is “could have”: मैं पास हो सकता था (main paas ho saktaa thaa), I could have passed. -ना था (-naa thaa) is “was supposed to”: मुझे कल आना था (mujhe kal aanaa thaa), I was supposed to come yesterday.