Cette règle s’applique aussi à ce cas.

Because règle is a feminine singular noun in French. The demonstrative adjective must agree with the noun:

• ce = masculine singular
• cet = masculine singular before a vowel or silent h
• cette = feminine singular
• ces = plural

So:

• ce cas = this case
• cette règle = this rule

Because the two nouns have different grammatical genders:

• cas is masculine
• règle is feminine

French determiners change to match the gender of the noun, even when English uses the same word this for both.

So:

• ce cas = this case
• cette règle = this rule

The verb is s’appliquer à, which means to apply to in this kind of sentence.

So:

• La règle s’applique à ce cas = The rule applies to this case.

It is reflexive in French because that is simply how this verb is commonly used for this meaning. You should learn s’appliquer à as a set expression.

Be careful not to translate word for word from English. French says something like the rule applies itself to this case, but in natural English we just say the rule applies to this case.

Se becomes s’ before a vowel sound for ease of pronunciation.

Since applique begins with a, French shortens:

• se applique → s’applique

This is called elision.

You see the same thing in many common forms:

• je aime → j’aime
• le ami → l’ami
• ne est pas → n’est pas

Because the verb pattern is s’appliquer à quelque chose = to apply to something.

So the preposition à is required with this meaning:

• Cette règle s’applique à ce cas.
• La loi s’applique à tous.

English uses to, but French uses à.

It is best to memorize the whole structure:

• s’appliquer à + noun/pronoun

Aussi means also or too.

In this sentence, it tells us that the rule applies to this case as well.

French often places aussi after the conjugated verb:

• Cette règle s’applique aussi à ce cas.

That is a very natural position. In some contexts, French may place aussi elsewhere, but here this placement is standard and clear.

A close alternative is:

• Cette règle s’applique également à ce cas.

Here également also means also / equally / as well, and it sounds a little more formal.

It is in the present tense: il/elle s’applique.

The subject is Cette règle, which is third person singular, so the verb is conjugated accordingly:

• je m’applique
• tu t’appliques
• il/elle s’applique
• nous nous appliquons
• vous vous appliquez
• ils/elles s’appliquent

In this sentence, the present tense expresses a general truth or current validity: this rule applies.

Here, cas is singular, because it follows ce:

• ce cas = this case

The tricky thing is that cas is spelled the same in the singular and the plural:

• un cas = a case
• des cas = cases

So you usually know whether it is singular or plural from the article or determiner:

• ce cas = singular
• ces cas = plural

Not in this sentence.

• à ce cas is used because it depends on the verb s’appliquer à
• dans ce cas means in that case / in this case as an expression of situation or consequence

Compare:

• Cette règle s’applique à ce cas. = This rule applies to this case.
• Dans ce cas, il faut attendre. = In that case, we must wait.

So à ce cas and dans ce cas do not mean the same thing.

A careful pronunciation would be approximately:

Set reh-gl sah-pleek oh-see ah suh kah

A few useful points:

• Cette sounds like set
• règle has one syllable in normal speech, roughly reh-gl
• s’applique sounds like sah-pleek
• aussi sounds like oh-see
• cas sounds like kah; the final s is silent

Also, French flows smoothly from word to word, so the whole sentence sounds connected rather than heavily separated.