Wenn ich die Einzahl kenne, finde ich die Mehrzahl oft schneller.

They are grammar terms:

• die Einzahl = singular
• die Mehrzahl = plural

In everyday German, learners will more often hear der Singular and der Plural, but Einzahl and Mehrzahl are perfectly understandable and are used especially in grammar explanations.

Because both nouns are feminine singular:

• die Einzahl
• die Mehrzahl

In this sentence, they are also in the accusative case, but for feminine singular, the article stays die in both nominative and accusative.

So:

• nominative: die Einzahl
• accusative: die Einzahl

The same is true for die Mehrzahl.

Because the first part begins with wenn, which creates a subordinate clause.

In German subordinate clauses, the conjugated verb usually goes to the end:

• Wenn ich die Einzahl kenne, ...

That is a very common German pattern.

Compare:

• Main clause: Ich kenne die Einzahl.
• Subordinate clause: ..., wenn ich die Einzahl kenne.

Because the second part is a main clause, and in German main clauses the finite verb normally comes in second position.

So in:

• finde ich die Mehrzahl oft schneller

the verb finde comes first in the clause because the wenn-clause has taken the first position overall:

1. Wenn ich die Einzahl kenne
2. finde
3. ich ...

This is a standard verb-second pattern in German.

Wenn is used for if in conditions and also for repeated or general situations meaning whenever.

Here it expresses a general idea:

• Wenn ich die Einzahl kenne, finde ich die Mehrzahl oft schneller.
• If I know the singular, I often find the plural faster.

It is not talking about one single past event. It is talking about a general pattern.

Yes, in many contexts wenn can mean either if or when/whenever, depending on the situation.

In this sentence, the feeling is close to:

• Whenever I know the singular, I often find the plural faster

or

• If I know the singular, I often find the plural faster

Because this is a general rule-like statement, English can naturally use if.

Schneller is the comparative form of schnell:

• schnell = fast / quickly
• schneller = faster / more quickly

Here it functions adverbially, describing how quickly the speaker can find the plural.

The comparison is usually understood from context: knowing the singular helps the speaker find the plural faster than otherwise or faster than before.

Because here schneller is being used like an adverb, not as an adjective before a noun.

Compare:

• ein schneller Hund = a fast dog
  
  • here schneller is an adjective modifying Hund, so it gets an ending
• Ich lese schneller. = I read faster.
  
  • here schneller describes the action, so it does not get an adjective ending

In your sentence, schneller describes finde, so it has no adjective ending.

Oft means often. It shows frequency.

So the sentence does not mean that knowing the singular always helps. It means it helps often.

German word order often puts adverbs like oft before another adverb such as schneller:

• ... finde ich die Mehrzahl oft schneller.

That sounds natural in German.

Because the sentence starts with the wenn-clause, and that whole clause counts as the first element of the sentence.

German main clauses then put the verb in second position:

• Wenn ich die Einzahl kenne, finde ich ...

If you start with the main clause instead, you get:

• Ich finde die Mehrzahl oft schneller, wenn ich die Einzahl kenne.

Both are correct. The word order changes because of the verb-second rule.

Not necessarily in the physical sense of to find something somewhere.

Here finden can mean something like:

• to work out
• to identify
• to come up with

So die Mehrzahl finden can mean to figure out the plural form.

That is a natural use of finden in German.

Yes, and many learners will hear those forms more often:

• Wenn ich den Singular kenne, finde ich den Plural oft schneller.

That version is also very natural.

Notice that the articles change because these nouns have different genders:

• der Singular → accusative den Singular
• der Plural → accusative den Plural

So the original sentence uses native German grammar terms, while this version uses the international terms.

Because in German, all nouns are capitalized.

So:

• Einzahl
• Mehrzahl

must begin with capital letters.

This is a basic but very important German spelling rule.

It sounds like a general statement or a habitual observation.

The present tense in both clauses supports that:

• Wenn ich ... kenne
• finde ich ...

So the meaning is something like:

• Whenever I know the singular, I often find the plural faster
• In general, knowing the singular often helps me get the plural more quickly

It is not tied to one particular moment unless the wider context makes it specific.