Основание ракетки оказалось лёгким.

Because ракетки depends on основание and means of the racket.

So the structure is:

• основание = base / foundation
• ракетки = of the racket

This is a very common Russian pattern: one noun is followed by another noun in the genitive to show possession or relationship.

Examples:

• крышка коробки = the lid of the box
• дверь дома = the door of the house
• основание ракетки = the base of the racket

So ракетки is not the subject. It is just modifying основание.

There are two main clues:

1. Основание is in the nominative form.
2. The verb оказалось agrees with it in neuter singular.

In Russian past tense, the verb agrees with the subject in gender and number:

• masculine: оказался
• feminine: оказалась
• neuter: оказалось
• plural: оказались

Since основание is a neuter singular noun, the verb is оказалось.

Оказалось means something like:

• turned out to be
• proved to be

It suggests that the speaker learned or discovered this fact, often unexpectedly.

So:

• Основание ракетки было лёгким = The base of the racket was light
  This simply states a fact.

• Основание ракетки оказалось лёгким = The base of the racket turned out to be light
  This adds the idea of discovery or result.

So оказалось is not just a neutral was. It tells you that the speaker found out this was the case.

Because Russian past-tense verbs agree with the subject.

The subject here is основание, and основание is a neuter noun. So the verb must also be neuter:

• основание оказалось
• система оказалась
• стол оказался
• предметы оказались

This is very normal in Russian and is one of the first things English speakers notice, because English past-tense verbs do not change for gender.

Because after оказаться, Russian usually puts the predicate noun or adjective in the instrumental case.

So:

• оказалось лёгким
• оказался трудным
• оказалась полезной
• оказались дорогими

This is a standard pattern:

• X оказался Y-ом / Y-ой / Y-им / Y-ыми
• X turned out to be Y

So лёгким is the instrumental singular form of лёгкий.

This is one of the most important grammar points in the sentence.

Yes. It agrees with основание in number and, in meaning, in gender, but it is also in the instrumental case because of оказалось.

Here is what is happening:

• основание = neuter singular
• therefore the adjective must also be singular and refer to a neuter noun
• but after оказалось, the adjective takes the instrumental

So we get:

• nominative neuter singular: лёгкое
• instrumental singular: лёгким

A useful detail: in the instrumental singular, masculine and neuter adjectives often have the same form:

• masculine: лёгким
• neuter: лёгким

So the form itself does not visually show neuter here, but the agreement is still determined by основание.

Yes, оказаться is a reflexive verb form, but here you should learn it as a whole vocabulary item.

In this sentence, оказаться does not literally mean something like to show oneself in a transparent, word-for-word way. In modern Russian, it functions as a set verb meaning:

• to turn out
• to prove to be

So it is best to memorize:

• оказаться + instrumental = to turn out to be

The -ся is part of the normal dictionary form of this verb.

The dictionary form is оказаться.

This sentence uses the past tense form:

• infinitive: оказаться
• past neuter singular: оказалось

Related forms:

• оказался = masculine
• оказалась = feminine
• оказалось = neuter
• оказались = plural

There is also an imperfective partner:

• оказываться

Very roughly:

• оказаться = perfective, turn out
• оказываться = imperfective, be turning out / turn out repeatedly / end up being

Yes. Russian word order is more flexible than English word order, because case endings show grammatical roles.

The version you have is fairly neutral:

• Основание ракетки оказалось лёгким.

But other orders are possible for emphasis, for example:

• Лёгким оказалось основание ракетки.
  Emphasis on light

• Оказалось лёгким основание ракетки.
  More literary or context-driven

Even when the order changes, the cases still tell you what each word is doing:

• основание = subject
• ракетки = dependent genitive
• лёгким = predicate adjective in instrumental

Основание literally means base, foundation, or bottom/supporting part, depending on context.

In this sentence, it refers to the base part of the racket. The exact natural English wording can vary depending on what kind of racket and what part is being described.

For a learner, the key point is grammatical:

• основание is a neuter noun
• it is the head noun of the phrase
• ракетки specifies what base it is

Yes, it matters for pronunciation and sometimes for meaning.

• лёгкий is pronounced with yo: roughly LYOH-kee
• лёгким is roughly LYOHK-im

In printed Russian, ё is often replaced by е, so you may sometimes see легким instead of лёгким. But the correct pronunciation is still with ё.

So even if a text writes легким, you should understand it as лёгким here.

Yes, and that is a very good idea.

A useful pattern is:

• [noun in nominative] + оказался / оказалась / оказалось / оказались + [adjective or noun in instrumental]

Examples:

• Экзамен оказался трудным. = The exam turned out to be difficult.
• Задача оказалась простой. = The task turned out to be simple.
• Место оказалось удобным. = The place turned out to be convenient.
• Основание ракетки оказалось лёгким.

This pattern is extremely common in Russian.