यह लाल सेब अच्छा है।

यह means this and is used for something near the speaker.

So in यह लाल सेब अच्छा है, यह points to a specific apple: this red apple.

A useful contrast:

• यह = this
• वह = that

In everyday speech, you may also hear ये used where careful written Hindi would use यह, but यह is the standard form to learn first.

In Hindi, an adjective usually comes before the noun when it directly describes it.

So:

• लाल सेब = red apple
• अच्छा सेब = good apple

That means:

• यह लाल सेब अच्छा है = This red apple is good

But if you say:

• यह सेब लाल है = This apple is red

So the position of लाल changes the structure:

• लाल सेब = red apple
• सेब लाल है = the apple is red

अच्छा means good, and here it describes सेब.

Hindi adjectives often agree with the noun in gender and number.
सेब is treated as masculine singular, so the adjective appears as अच्छा.

Compare:

• अच्छा = masculine singular
• अच्छी = feminine singular
• अच्छे = masculine plural / some other contexts

So:

• यह सेब अच्छा है = This apple is good
• यह किताब अच्छी है = This book is good

Because लाल is an indeclinable adjective in normal use. That means it usually does not change form for gender or number.

So you get:

• लाल सेब = red apple
• लाल किताब = red book
• लाल सेब = red apples

But अच्छा is a changeable adjective, so it agrees with the noun:

• अच्छा सेब
• अच्छी किताब
• अच्छे सेब

This is a very common thing in Hindi:
some adjectives change, and some do not.

है is the present-tense form of to be: is.

So:

• अच्छा है = is good

Hindi often puts the main descriptive word before है, so the pattern is:

• [subject] + [description] + है

Here:

• यह लाल सेब = this red apple
• अच्छा है = is good

Together: This red apple is good.

Hindi does not normally use articles like English a, an, and the.

So a noun like सेब can mean:

• an apple
• the apple
• just apple

The exact meaning depends on context.

In this sentence, यह already makes it specific:

• यह लाल सेब = this red apple

So Hindi does not need a separate word for the here.

Not naturally. In this sentence, लाल is attached directly to सेब, so it means red apple.

Structure:

• यह [लाल सेब] अच्छा है
• This [red apple] is good

If you want to say This apple is red, you would normally say:

• यह सेब लाल है = This apple is red

If you want This apple is red and good, you could say something like:

• यह सेब लाल और अच्छा है।

So adjective placement matters a lot.

सेब is masculine.

Sometimes you can guess gender from patterns, but often with Hindi nouns you simply have to learn the gender with the word.

In this sentence, one clue is the adjective agreement:

• सेब takes अच्छा
• so it is masculine singular here

That is why:

• यह सेब अच्छा है not
• यह सेब अच्छी है

The main change would be in the adjective that agrees.

For example, with किताब (book, feminine):

• यह लाल किताब अच्छी है।
• This red book is good.

Notice:

• लाल stays the same
• अच्छा changes to अच्छी

So this sentence is a good example of two different adjective types working together:

• लाल = unchanged
• अच्छा / अच्छी / अच्छे = changes to match the noun

A simple learner-friendly pronunciation is:

yeh laal seb achchhaa hai

A few notes:

• यह is often pronounced roughly like yeh
• लाल sounds like laal
• सेब sounds like seb (similar to sayb, but shorter and cleaner)
• अच्छा sounds like ach-chhaa
• है is often pronounced like hai

So the full rhythm is:

yeh laal seb achchhaa hai

In very casual speech, Hindi speakers sometimes drop forms of to be in certain situations, especially in conversation. But for learners, and especially in standard written Hindi, you should keep है here.

So the correct standard sentence is:

• यह लाल सेब अच्छा है।

That is the form you should learn and use first.