अभ्यास करते समय यह नियम ज़रूरी है।

Here अभ्यास is a noun meaning practice, exercise, or rehearsal, depending on context.

In this sentence, it is part of the very common expression अभ्यास करना, which means to practice. So Hindi is not using a single verb for practice here; it is using a noun + करना.

This is a common Hindi pattern:

verb + ते समय = while doing, when doing, during the act of doing

Since अभ्यास is a noun, Hindi first builds the idea to practice as अभ्यास करना. Then it turns that into:

अभ्यास करते समय = while practicing

So the structure is essentially:

• अभ्यास = practice
• करते = doing
• समय = time

Literally, it is something like at the time of doing practice, but in natural English it is just while practicing.

In this construction, Hindi normally uses the -ते form before समय:

• पढ़ते समय = while reading
• खाते समय = while eating
• चलते समय = while walking
• करते समय = while doing

So करते समय is best learned as a fixed pattern meaning while doing.

For a beginner, the simplest rule is:

V-ते समय = while V-ing

The -ते here does not mean plural in the way it might seem to at first glance. It is just the normal form used in this expression.

Because V-ते समय is already a complete and established expression meaning while doing V or at the time of doing V.

So:

• अभ्यास करते समय = correct
• अभ्यास करते समय में = not natural here

Hindi often uses समय this way without any extra postposition in such expressions.

Because यह is the direct form, and इस is the oblique form.

You use यह when there is no postposition attached:

• यह नियम = this rule

You use इस when a postposition follows:

• इस नियम को
• इस नियम में
• इस नियम का

Since नियम here has no postposition after it, यह नियम is the correct form.

नियम is the main noun being talked about. In simple terms, it is the subject of the sentence.

The sentence breaks down like this:

• अभ्यास करते समय = while practicing
• यह नियम = this rule
• ज़रूरी है = is necessary

So the basic core is:

यह नियम ज़रूरी है = this rule is necessary

And अभ्यास करते समय adds the context.

Because ज़रूरी is an indeclinable adjective in normal usage. That means it usually stays the same regardless of gender or number.

For example:

• यह नियम ज़रूरी है
• यह बात ज़रूरी है
• ये नियम ज़रूरी हैं
• ये बातें ज़रूरी हैं

Notice that ज़रूरी stays the same each time. What changes instead is usually the verb:

• है for singular
• हैं for plural

Because Hindi usually needs a form of to be in sentences like this.

Here the structure is:

यह नियम + ज़रूरी + है

That is the normal Hindi way to say:

This rule is necessary

In very casual notes, titles, or headlines, Hindi may sometimes drop है, but in a normal full sentence it should be there.

Yes, Hindi word order is more flexible than English word order.

The given sentence is very natural:

अभ्यास करते समय यह नियम ज़रूरी है।

But you could also say:

यह नियम अभ्यास करते समय ज़रूरी है।

Both are grammatical. The difference is mainly in emphasis:

• अभ्यास करते समय... puts the time/context first
• यह नियम... puts the rule first

The original sentence sounds very natural because it starts by setting the situation.

Yes, you could, and the meaning would be very similar.

• अभ्यास के समय यह नियम ज़रूरी है।
• अभ्यास करते समय यह नियम ज़रूरी है।

The difference is subtle:

• अभ्यास के समय = during practice, at practice time
• अभ्यास करते समय = while actually practicing

The version with करते समय feels a little more verbal and action-based, while के समय feels a little more noun-based.

Both can mean necessary, but the tone is different.

• ज़रूरी is very common and natural in everyday Hindi
• आवश्यक is more formal and more bookish or official

So this sentence with ज़रूरी sounds natural in normal speech:

अभ्यास करते समय यह नियम ज़रूरी है।

A more formal version could be:

अभ्यास करते समय यह नियम आवश्यक है।

That dot is called a nukta. It changes the sound of the letter.

• ज is usually a j sound
• ज़ is used for a z sound

So ज़रूरी is ideally pronounced with a z sound.

That said, in everyday speech many speakers pronounce words like this closer to जरूरी, and that is very common too. So you may hear both.

You would usually change यह to ये and है to हैं:

अभ्यास करते समय ये नियम ज़रूरी हैं।

So the main agreement change is in the verb:

• singular: यह नियम ज़रूरी है
• plural: ये नियम ज़रूरी हैं

Notice that ज़रूरी still does not change.