Tämä mandariini on makea ja helppo kuoria.

Tämä means this.

In Tämä mandariini, it works like this in English: this mandarin.

It comes first because Finnish often starts a simple sentence with the subject:

• Tämä mandariini = the subject
• on = is
• makea ja helppo kuoria = sweet and easy to peel

So the structure is very close to English:

• This mandarin is sweet and easy to peel.

Mandariini is in the nominative singular, which is the basic dictionary form.

It appears in this form because it is the subject of the sentence:

• Tämä mandariini = This mandarin

In simple X is Y sentences, the subject is normally in the nominative:

• Talo on suuri. = The house is big.
• Omena on punainen. = The apple is red.
• Tämä mandariini on makea. = This mandarin is sweet.

On is the third person singular form of the verb olla, which means to be.

So:

• on = is

Examples:

• Hän on opettaja. = He/she is a teacher.
• Kahvi on kuumaa. = The coffee is hot.
• Tämä mandariini on makea. = This mandarin is sweet.

Here makea is a predicate adjective describing the subject tämä mandariini.

Because the subject is singular and in the nominative, the adjective is also in its basic nominative form:

• mandariini on makea = the mandarin is sweet

Compare:

• Keitto on kuumaa. = The soup is hot.
• Sää on kylmä. = The weather is cold.

Finnish predicate adjectives do not always behave exactly like English adjectives, and learners often notice forms like kuumaa. But with a countable singular noun like mandariini, makea is the expected form here.

Because both are adjectives describing the subject tämä mandariini.

The sentence has two linked descriptions:

• makea = sweet
• helppo kuoria = easy to peel

So the pattern is:

• This mandarin is sweet
• This mandarin is easy to peel

The word ja simply joins the two descriptions:

• makea ja helppo kuoria = sweet and easy to peel

Helppo kuoria means easy to peel.

This is a very common Finnish pattern:

• adjective + infinitive

Examples:

• helppo ymmärtää = easy to understand
• vaikea selittää = difficult to explain
• mukava tavata = nice to meet
• helppo kuoria = easy to peel

So in your sentence:

• Tämä mandariini on helppo kuoria.
• Literally: This mandarin is easy to peel.

Because after helppo in this kind of structure, Finnish normally uses the first infinitive.

The dictionary form of the verb is:

• kuoria = to peel

So:

• helppo kuoria = easy to peel

Other forms would mean different things:

• kuorimaan = to peel / into peeling / go peel, depending on the structure
• kuorittu = peeled
• kuoriminen = peeling

Only kuoria fits this pattern naturally:

• Tämä mandariini on helppo kuoria.

In this sentence, the doer is generic or unspecified, just like in English easy to peel.

The focus is on the mandarin, not on the person peeling it.

So:

• Tämä mandariini on helppo kuoria
  means
• This mandarin is easy (for someone) to peel

Finnish does this very often. The understood meaning is general:

• anyone
• a person
• people in general

You do not need to add a subject like someone unless it is important.

Yes.

This is one of the trickiest but most useful things in the sentence.

In helppo kuoria, the verb kuoria does not show its object separately, because the object is understood to be the same thing as the subject of the whole sentence:

• Tämä mandariini on helppo kuoria.
• The mandarin is the thing that gets peeled.

So even though mandariini is the subject of on, it is also understood as the object of kuoria in meaning.

English works similarly:

• This book is easy to read.
• This shirt is hard to wash.
• This mandarin is easy to peel.

Because Finnish infinitives do not need a separate word like English to.

In English:

• to peel

In Finnish:

• kuoria

So the Finnish infinitive already includes the idea that English expresses with to.

Examples:

• syödä = to eat
• juoda = to drink
• kuoria = to peel

That is why helppo kuoria is enough for easy to peel.

Yes, a little.

The neutral, most natural order here is:

• Tämä mandariini on makea ja helppo kuoria.

But you could also say:

• Mandariini on makea ja helppo kuoria. = The mandarin is sweet and easy to peel.
• Tämä on makea ja helppo kuoria oleva mandariini. = grammatically possible in parts, but awkward and not the normal way to say it

Finnish word order is more flexible than English, but not every rearrangement sounds natural. For a learner, the original sentence is the best model.

You probably could, but it would sound much less natural in everyday speech.

• helppo kuoria = simple, common, natural
• helposti kuorittava = more formal, more technical, more like easily peelable

In ordinary Finnish, people strongly prefer:

• Tämä mandariini on helppo kuoria.

So if your goal is normal spoken or written Finnish, helppo + infinitive is the better choice.

Yes — very much so.

This pattern is extremely useful:

• [noun] + on + adjective + infinitive

Examples:

• Tämä kirja on helppo lukea. = This book is easy to read.
• Tämä sana on vaikea lausua. = This word is hard to pronounce.
• Nämä kengät ovat mukava käyttää. = These shoes are comfortable to wear.
  
  • Many speakers would more naturally say mukavat käyttää or restructure slightly, but the pattern idea is the same.
• Tämä omena on hyvä syödä. = This apple is good to eat.

So Tämä mandariini on makea ja helppo kuoria gives you a very practical model for building your own sentences.