Αυτό το καινούριο πουκάμισο είναι ωραίο, αλλά ο γιακάς του είναι πολύ σκληρός.

Because Greek often uses both:

• αυτό = this
• το = the

So Αυτό το πουκάμισο literally means this the shirt, but in natural English we simply say this shirt.

This is normal in Greek. The demonstrative (αυτός, αυτή, αυτό) usually appears together with the definite article (ο, η, το):

• αυτό το βιβλίο = this book
• αυτή η πόρτα = this door
• αυτό το πουκάμισο = this shirt

Because πουκάμισο is a neuter singular noun, so the adjective must match it in gender, number, and case.

• πουκάμισο = neuter singular
• καινούριο = neuter singular form of καινούριος = new

So:

• καινούριος = masculine
• καινούρια / καινούργια = feminine
• καινούριο / καινούργιο = neuter

Since πουκάμισο is neuter, καινούριο is the correct form.

ωραίο is the neuter singular form of ωραίος, and it means something like:

• nice
• beautiful
• lovely
• good-looking

In this sentence, Το πουκάμισο είναι ωραίο means The shirt is nice / looks nice.

Again, it is ωραίο because it refers to πουκάμισο, which is neuter.

Compare:

• ο άντρας είναι ωραίος = the man is handsome
• η γυναίκα είναι ωραία = the woman is beautiful
• το πουκάμισο είναι ωραίο = the shirt is nice

Yes. Many Greek nouns ending in -ο are neuter, and πουκάμισο is one of them.

So:

• το πουκάμισο = the shirt

Its neuter gender affects the words around it:

• αυτό not αυτός / αυτή
• το not ο / η
• καινούριο
• ωραίο

Gender in Greek is grammatical, so it does not always match natural gender. Objects like shirts can be masculine, feminine, or neuter depending on the noun, and πουκάμισο happens to be neuter.

Literally, it means the collar of it.

• ο γιακάς = the collar
• του = of it / its

So:

• ο γιακάς του είναι πολύ σκληρός = its collar is very stiff

Greek often expresses possession this way, using the definite article plus a weak possessive form:

• το σπίτι μου = my house
• ο φίλος της = her friend
• ο γιακάς του = its collar / his collar

Here του refers back to πουκάμισο.

Because του is the genitive singular form used for:

• his
• its
• also of him / of it

Since πουκάμισο is singular, του is the correct form for its.

Examples:

• το χρώμα του = its color / his color
• το μανίκι του = its sleeve / his sleeve
• ο γιακάς του = its collar

Even though πουκάμισο is neuter, the possessive clitic is still του.

Because σκληρός describes γιακάς, while ωραίο describes πουκάμισο.

• πουκάμισο is neuter, so: ωραίο
• γιακάς is masculine, so: σκληρός

Greek adjectives must agree with the noun they describe.

So in this sentence:

• το πουκάμισο είναι ωραίο
• ο γιακάς είναι σκληρός

That is why the endings are different.

Yes, σκληρός basically means hard, stiff, or harsh, depending on context.

For a collar, πολύ σκληρός most naturally means:

• very stiff
• possibly too hard/rigid

So the idea is that the shirt looks nice, but the collar is uncomfortable because it is stiff.

πολύ means very here.

• πολύ σκληρός = very stiff
• πολύ ωραίο = very nice

In this use, πολύ is an adverb and does not change form.

It can also mean much / a lot, depending on context, but here it clearly means very.

The sentence as given is very natural, but Greek word order is more flexible than English.

Standard version:

• Αυτό το καινούριο πουκάμισο είναι ωραίο, αλλά ο γιακάς του είναι πολύ σκληρός.

You could also hear slight variations for emphasis, such as:

• Αυτό το καινούριο πουκάμισο είναι ωραίο, αλλά είναι πολύ σκληρός ο γιακάς του.

That sounds a bit more emphatic on πολύ σκληρός.

Still, the original version is probably the clearest and most neutral for a learner.

Greek normally keeps είναι in both clauses here:

• το πουκάμισο είναι ωραίο
• ο γιακάς του είναι πολύ σκληρός

So repeating είναι is completely normal and expected.

In very informal speech, Greek sometimes drops forms of to be in certain contexts, but in a sentence like this, learners should definitely keep both verbs.

No, Greek has more than one common word for new.

Two very common choices are:

• καινούριος / καινούργιος
• νέος

In many everyday contexts, both can mean new.

So:

• καινούριο πουκάμισο
• νέο πουκάμισο

Both can mean new shirt.

Very roughly:

• καινούριος often strongly means brand-new / newly bought
• νέος can also mean new in a broader sense, and for people it means young

So καινούριο πουκάμισο is a very natural choice here.

γιακάς is pronounced roughly ya-KAS, with the stress on the last syllable.

The accent mark in Greek shows which syllable is stressed:

• γιακάς
• ωραίο
• καινούριο

Stress matters in Greek pronunciation, so learners should pay attention to these marks.

A rough pronunciation guide for the whole sentence could be:

• af-TO to ke-NU-rio pu-KA-mi-so I-ne o-RE-o, a-LA o ya-KAS tu I-ne po-LI skli-ROS

This is only approximate, but it helps show the stressed syllables.

It has two simple clauses joined by αλλά = but.

1. Αυτό το καινούριο πουκάμισο είναι ωραίο
   = This new shirt is nice.

2. ο γιακάς του είναι πολύ σκληρός
   = its collar is very stiff.

So the pattern is:

• subject + είναι + adjective
• subject + είναι + adverb + adjective

More specifically:

• Αυτό το καινούριο πουκάμισο = subject noun phrase
• είναι ωραίο = is nice
• αλλά = but
• ο γιακάς του = the collar of it / its collar
• είναι πολύ σκληρός = is very stiff

It is a very useful model sentence for learning:

• demonstratives
• articles
• adjective agreement
• possession with του
• contrast with αλλά