यह शब्दकोश उपयोगी है।

यह means this here.

Its careful written pronunciation is often given as yah, but in everyday speech it is very often pronounced more like yeh. So learners will commonly hear something close to yeh shabdkosh upyogi hai.

A useful thing to know:

• यह = this
• वह = that

Hindi usually follows Subject–Object–Verb order, and in simple equational sentences like this one, the verb usually comes at the end.

So the structure is:

• यह = this
• शब्दकोश = dictionary
• उपयोगी = useful
• है = is

Literally, it is closer to:

This dictionary useful is.

That is completely normal in Hindi.

शब्दकोश is the Hindi word for dictionary.

It is made from:

• शब्द = word
• कोश = collection, treasury, lexicon

So the idea is basically a collection of words.

This is a fairly formal and standard word for dictionary.

शब्दकोश is generally treated as masculine.

In this sentence, though, that does not create a very visible change, because उपयोगी is an adjective that usually does not change form for masculine/feminine in this kind of usage.

The verb है also stays the same here because the subject is singular.

So even if you are aware of gender, the sentence still remains:

यह शब्दकोश उपयोगी है।

Usually, उपयोगी is used as an invariable adjective, meaning it often stays the same in form.

So you can say:

• यह शब्दकोश उपयोगी है। = This dictionary is useful.
• यह किताब उपयोगी है। = This book is useful.
• ये शब्दकोश उपयोगी हैं। = These dictionaries are useful.

Notice that उपयोगी stays उपयोगी.

The main change for plural is usually in the verb:

• singular: है
• plural: हैं

In normal standard Hindi, है is needed here because it is the present-tense form of to be.

So:

• यह शब्दकोश उपयोगी है। = This dictionary is useful.

If you remove है, the sentence may sound incomplete in standard prose, although in headings, notes, slogans, poetry, or very casual contexts, Hindi sometimes drops forms of to be.

For a learner, the safest rule is:

• in full sentences like this, keep है

Hindi does not have articles that work exactly like English a/an and the.

So शब्दकोश can mean:

• a dictionary
• the dictionary

The exact sense depends on context.

In this sentence, यह already points to a specific thing, so the meaning is naturally this dictionary.

You would usually change:

• यह → ये
• है → हैं

So:

ये शब्दकोश उपयोगी हैं।
= These dictionaries are useful.

A very important learner note:

• In modern standard written Hindi, यह and वह are often used for singular, while ये and वे are used for plural.
• In everyday speech, people very often say ये even for singular this.

Here, यह is functioning like a demonstrative adjective because it is directly describing the noun शब्दकोश:

• यह शब्दकोश = this dictionary

But यह can also be a pronoun:

• यह उपयोगी है। = This is useful.

So the same word can act like this in both senses, depending on whether a noun follows it.

A natural pronunciation would be close to:

yeh shabdkosh upyogi hai

A rough breakdown:

• यह → yeh
• शब्दकोश → shabd-kosh
• उपयोगी → u-pyo-gi or up-yo-gi
• है → hai (like high, but shorter and lighter)

A natural rhythm is:

yeh SHABD-kosh up-YO-gi hai

Yes. That means:

This dictionary is very useful.

You are just adding बहुत = very.

So:

• यह शब्दकोश उपयोगी है। = This dictionary is useful.
• यह शब्दकोश बहुत उपयोगी है। = This dictionary is very useful.

That is a very common and natural expansion of the sentence.