Na jaře se mi nejvíc líbí náš park, protože tam je hodně květin.

For seasons, Czech uses fixed combinations of preposition + case that you mostly have to memorize:

• na jaře – in (the) spring
• v létě – in (the) summer
• na podzim – in (the) autumn / fall
• v zimě – in (the) winter

So na jaře is just the natural idiomatic way to say in spring.
A phrase like v jaru sounds archaic or wrong in modern Czech.

Jaře is the locative singular of jaro (spring).

Very simplified singular forms of jaro:

• Nominative (dictionary form): jaro
• Genitive: jara
• Dative: jaru
• Accusative: jaro
• Locative: (na) jaře
• Instrumental: jarem

The preposition na in a time expression like na jaře requires the locative case, which is why you see jaře instead of jaro or jaru.

The verb here is líbit se, a reflexive verb that means to be pleasing.

• se is the reflexive particle that belongs to the verb líbit se.
• mi is the dative form of já (I), meaning to me.

So:

• Náš park se mi líbí literally = Our park is pleasing to me.
  (The park is the grammatical subject; me is in the dative.)

English normally says I like our park, but Czech structures it like Our park pleases me.

You always use líbit se with a dative person:

• Líbí se mi ten film. – That film pleases me / I like that film.
• Líbí se ti to město? – Does that city please you? / Do you like that city?

Yes, both orders are correct and mean the same thing in most contexts:

• Se mi nejvíc líbí náš park.
• Náš park se mi nejvíc líbí.

Czech word order is quite flexible. The differences are mainly in emphasis:

• Starting with Náš park puts more emphasis on our park as the topic.
• Starting with Se mi nejvíc líbí makes the degree of liking (most) more prominent, with náš park coming as the new/important information at the end.

In everyday speech both orders are natural; learners can safely use Náš park se mi nejvíc líbí na jaře as a very transparent structure.

Both can be translated as I like our park, but they have slightly different nuances:

• líbí se mi náš park – literally our park pleases me.

  • Focuses on your impression, how nice/pleasant it seems (visually, atmospherically, etc.).
  • Often used for things you see, hear, taste, experience: films, cities, clothes, food, music.

• mám rád náš park – literally I have our park as dear.

  • More about affection / fondness.
  • Common with people, animals, activities, and also with places and things.

In this sentence, talking about spring and flowers, líbí se mi náš park fits very well because you’re describing how pleasant/nice the park is at that time.

Nejvíc is the superlative of víc (more) and corresponds to English the most.

• hodně – a lot / much / many
• víc (více) – more
• nejvíc (nejvíce) – (the) most

nejvíc vs nejvíce:

• Meaning: the same.
• Style:
  
  • nejvíc – more common in everyday speech, informal.
  • nejvíce – a bit more formal or written style.

Typical positions:

• Na jaře se mi nejvíc líbí náš park.
• Na jaře se mi líbí náš park nejvíc.

Both are correct; moving nejvíc changes only the emphasis slightly. Putting it right before líbí is very common.

The possessive pronoun náš has to agree with the gender and number of the noun.

• park is masculine singular.
• Masculine singular nominative form of our is náš.

Very simplified nominative forms of náš:

• Masculine singular: náš
• Feminine singular: naše
• Neuter singular: naše
• Masculine animate plural: naši
• All other plurals: naše

So you say:

• náš park (masc.) – our park
• naše zahrada (fem.) – our garden
• naše auto (neut.) – our car
• naše parky (pl.) – our parks
• naši kamarádi (masc. animate pl.) – our (male) friends

Protože introduces a subordinate clause (a reason clause: because …). In Czech, you normally put a comma before conjunctions like protože, když, jestli, že, when they start a subordinate clause.

So:

• Na jaře se mi nejvíc líbí náš park, protože tam je hodně květin.

The comma boundary is: [main clause], protože [subordinate clause].
In standard Czech this comma is required, just like in English I like our park, because there are a lot of flowers there.

Yes, you can say both:

• protože tam je hodně květin
• protože je tam hodně květin

Both are grammatically correct and usually interchangeable.

Subtle nuance:

• tam je hodně květin can slightly emphasize the place (there):
  “…because there there are many flowers (as opposed to somewhere else).”
• je tam hodně květin is a very neutral word order.

In everyday conversation, protože je tam hodně květin is probably the most neutral-sounding version, but the sentence as given is perfectly natural too.

The word hodně (a lot of, many, much) is followed by the genitive case, usually genitive plural for countable nouns.

Basic noun:

• květina – a flower

Genitive plural:

• (bez) květin – (without) flowers

So:

• hodně květin – a lot of flowers / many flowers

If you said hodně květiny, that sounds like you are talking about some mass of one particular flower as a substance, which is not how you normally talk about flowers in a park. For counting “how many flowers”, Czech wants genitive plural after hodně:

• hodně květin – many flowers
• málo květin – few flowers
• mnoho květin – many flowers (more formal)

The base form (nominative singular) is květina (a flower).

květin here is genitive plural.

Very simplified paradigm:

• Nominative singular: květina – a flower
• Nominative plural: květiny – flowers
• Genitive plural: květin – of flowers

Words like hodně, málo, mnoho, spousta usually take a genitive noun after them, so with hodně you get hodně květin.

• tam = there (away from the speaker)
• tady = here (near the speaker)

In this sentence:

• protože tam je hodně květin – because there are a lot of flowers there
  (in that place you’re talking about – the park)

If you were physically in the park while speaking, you might say:

• protože tady je hodně květin – because there are a lot of flowers here.

Yes, that version is fully correct:

• Náš park se mi nejvíc líbí na jaře, protože je tam hodně květin.

Meaning-wise it is the same. The small differences:

• Starting with Náš park makes our park the clear topic of the sentence.
• je tam hodně květin is a bit more neutral word order than tam je hodně květin.

All of these are natural Czech:

• Na jaře se mi nejvíc líbí náš park, protože tam je hodně květin.
• Náš park se mi nejvíc líbí na jaře, protože je tam hodně květin.
• Na jaře se mi nejvíc líbí náš park, protože je tam hodně květin.

They all express the same idea; the differences are only in subtle emphasis and flow.