Je me sers de cette application pour apprendre le français.

Because the verb here is se servir de, which is a pronominal verb. In this expression, se servir de means to use.

So:

• je me sers de... = I use...
• tu te sers de... = you use...
• il/elle se sert de... = he/she uses...

If you said je sers by itself, that would usually come from the verb servir meaning to serve rather than to use.

So in this sentence:

• Je me sers de cette application = I use this app

Se servir de means to use, to make use of, or sometimes to rely on something as a tool.

It is a common and natural expression, though in everyday French many speakers also use utiliser:

• J’utilise cette application = I use this app
• Je me sers de cette application = I use this app

Both are correct.
Se servir de can sound a little more like making use of something.

Because the full expression is se servir de quelque chose.

So the pattern is:

• se servir de + noun
• se servir de + thing used

Examples:

• Je me sers d’un stylo. = I use a pen.
• Elle se sert de son ordinateur. = She uses her computer.
• Nous nous servons de cette application. = We use this app.

So the de is required by the verb expression.

De changes form depending on the word that follows.

• Before a consonant sound: de
• Before a vowel or silent h: d’

Here, cette application starts with a vowel sound, so in natural spelling and pronunciation you usually get:

• de cette application

But with an article like une application, it becomes:

• d’une application

Examples:

• Je me sers de cette application.
• Je me sers d’une application.
• Je me sers de mon téléphone.
• Je me sers d’un dictionnaire.

So whether you see de or d’ depends on the next word.

Because application is a feminine noun.

The demonstrative adjectives are:

• ce for masculine singular
• cet for masculine singular before a vowel sound
• cette for feminine singular
• ces for plural

Since application is feminine, French uses cette, even though it begins with a vowel.

So:

• cet ordinateur = this computer
• cette application = this app
• ces applications = these apps

Pour + infinitive often means in order to or for the purpose of doing something.

So:

• pour apprendre = to learn / in order to learn

In the full sentence:

• Je me sers de cette application pour apprendre le français.
• I use this app to learn French.

It explains the purpose of using the app.

Other examples:

• Je travaille pour gagner de l’argent. = I work to earn money.
• Elle étudie pour réussir. = She studies in order to succeed.

In French, names of languages are usually preceded by the definite article when they are the object of a verb like apprendre, parler, étudier, aimer, etc.

So French says:

• apprendre le français = to learn French
• parler anglais is sometimes heard informally, but the standard form is parler anglais? Actually, with parler, both forms can appear depending on structure, but parler le français means to speak the French language, often more specifically.

For this sentence, apprendre le français is the normal and expected form.

Other examples:

• J’apprends l’espagnol. = I’m learning Spanish.
• Elle étudie l’allemand. = She studies German.

The article changes depending on the language:

• le français
• l’anglais
• l’espagnol

In French, names of languages are not capitalized.

So you write:

• le français
• l’anglais
• l’espagnol

This is different from English, where language names are capitalized.

However, nationality words behave differently depending on how they are used:

• un Français = a Frenchman / French person → capitalized because it is a noun
• français = French (language or adjective) → not capitalized

So here:

• le français = the French language

A rough pronunciation guide for an English speaker is:

• Je ≈ zhuh
• me ≈ muh
• sers ≈ sair
• de ≈ duh
• cette ≈ set
• application ≈ a-plee-ka-syon

Very roughly together:

• zhuh muh sair duh set a-plee-ka-syon

A few useful points:

• j in je sounds like the s in measure
• r in sers is the French r
• tion in application sounds like syon
• In natural speech, some sounds may link smoothly together

If you want a more French-like rhythm, try saying it in groups:

• Je me sers / de cette application / pour apprendre le français

Yes. In French, application commonly means app in the technology sense.

So:

• une application = an app / application
• cette application = this app

In casual speech, people may also shorten it informally, but application is completely standard and natural.

Yes, absolutely.

• Je me sers de cette application pour apprendre le français.
• J’utilise cette application pour apprendre le français.

Both mean I use this app to learn French.

A small difference in feel:

• utiliser = straightforward to use
• se servir de = to use / make use of

In many contexts, they are interchangeable.

Here is the present tense:

• je me sers
• tu te sers
• il / elle / on se sert
• nous nous servons
• vous vous servez
• ils / elles se servent

Notice the reflexive pronouns:

• me, te, se, nous, vous, se

And remember that it is followed by de:

• Je me sers de mon téléphone.
• Nous nous servons de cette application.

Yes, but the original order is the most natural and neutral:

• Je me sers de cette application pour apprendre le français.

You could also emphasize the purpose in other ways, for example:

• Pour apprendre le français, je me sers de cette application.

That means:

• To learn French, I use this app.

Both are correct, but the original sentence is the most straightforward everyday version.