In Mathe fühle ich mich heute sicherer als gestern.

Because German often puts the topic or time/place/context first.

Here, In Mathe means in math / in maths class / when it comes to math. By putting it first, the speaker sets the context right away:

• In Mathe fühle ich mich heute sicherer als gestern.

This also causes the verb to come immediately after it, which is why you get fühle ich and not ich fühle.

German main clauses follow the verb-second rule:

• position 1: In Mathe
• position 2: fühle
• then: ich mich heute sicherer als gestern

You could also say:

• Ich fühle mich heute in Mathe sicherer als gestern.

That is also correct, but the original version gives In Mathe more emphasis.

Mathe is the common everyday short form of Mathematik.

So:

• Mathe = informal, normal in speech and everyday writing
• Mathematik = more formal, full word

Both are correct here:

• In Mathe fühle ich mich heute sicherer als gestern.
• In Mathematik fühle ich mich heute sicherer als gestern.

A learner will hear Mathe very often in school-related contexts.

Because the verb here is sich fühlen, which is a reflexive verb in German.

So German says:

• ich fühle mich = I feel
• literally: I feel myself

The reflexive pronoun changes with the subject:

• ich fühle mich
• du fühlst dich
• er/sie fühlt sich
• wir fühlen uns

In English, you usually just say I feel confident, but German normally uses the reflexive structure:

• Ich fühle mich sicher.

Without mich, the sentence would sound incomplete or wrong in this meaning.

Because sich fühlen takes the accusative reflexive pronoun, not the dative one.

So:

• ich fühle mich
• not: ich fühle mir

Compare:

• Ich wasche mich.
• Ich fühle mich sicher.

The accusative reflexive forms are:

• mich
• dich
• sich
• uns
• euch
• sich

So mich is correct because ich is the subject and sich fühlen uses accusative reflexive pronouns.

Because heute is a time expression, and in German time words often come before adjectives or other information later in the sentence.

So the sentence naturally flows like this:

• In Mathe — context
• fühle ich mich — verb + subject + reflexive pronoun
• heute — time
• sicherer — description
• als gestern — comparison

This order is very natural in German.

You can move things around for emphasis, but not every version sounds equally natural. For example:

• Ich fühle mich heute in Mathe sicherer als gestern.
• Heute fühle ich mich in Mathe sicherer als gestern.

These are also correct.

Here, sicherer means more confident or more sure of myself, not safer.

The basic adjective is:

• sicher = sure, certain, confident, safe

In this sentence, because it is about math, the meaning is:

• confident
• more confident than yesterday

So:

• Ich fühle mich sicher. = I feel confident / sure.
• Ich fühle mich sicherer. = I feel more confident.

German often uses sicher in contexts where English might prefer confident.

Sicherer is the comparative form of sicher.

Pattern:

• positive: sicher = confident
• comparative: sicherer = more confident
• superlative: am sichersten = most confident

For many German adjectives, you form the comparative by adding -er:

• klein → kleiner
• schnell → schneller
• sicher → sicherer

So German often uses a single comparative word where English uses more + adjective.

Because als is the word used after a comparative in German.

So:

• sicherer als gestern = more confident than yesterday

This is the normal pattern:

• größer als = bigger than
• besser als = better than
• interessanter als = more interesting than

A very common learner issue is mixing up als and wie:

• als = than, after a comparative
• wie = as, in equal comparisons

Examples:

• Ich bin größer als du. = I am taller than you.
• Ich bin so groß wie du. = I am as tall as you.

So here als is required because sicherer is comparative.

Because gestern works here as a compact time expression: than yesterday.

German, like English, can compare directly to a time point:

• heute sicherer als gestern
• today more confident than yesterday

You do not need a full clause such as:

• ... als ich gestern war

That fuller version is possible in some contexts, but it is heavier and unnecessary here.

So:

• heute sicherer als gestern = concise and natural
• literally: today more confident than yesterday

Because gestern is an adverb meaning yesterday.

It is not being used like a noun here, so it does not take:

• an article
• adjective endings
• noun case endings

Compare:

• heute = today
• gestern = yesterday
• morgen = tomorrow

These words often act as adverbs:

• Heute lerne ich.
• Gestern war ich müde.
• Morgen schreibe ich einen Test.

In als gestern, gestern simply means than yesterday.

Yes, that is possible, but it is not exactly the same in tone.

Compare:

• Ich fühle mich heute in Mathe sicherer als gestern.
  = I feel more confident in math today than yesterday.

• Ich bin heute in Mathe sicherer als gestern.
  = I am more confident in math today than yesterday.

Both can work, but sich fühlen emphasizes the speaker’s inner feeling or perception more clearly. That is why it is often the better choice when talking about confidence.

It can mean either, depending on context.

In Mathe is a very natural school-related expression and can mean:

• in math
• in maths
• in math class
• when it comes to math

German often uses school subjects this way:

• In Englisch bin ich gut.
• In Geschichte habe ich morgen einen Test.
• In Bio verstehe ich das Thema nicht.

So it does not have to refer only to the physical classroom. It can mean the whole subject area.

Yes. German word order is flexible, but the finite verb still has to stay in second position in a main clause.

Possible versions:

• In Mathe fühle ich mich heute sicherer als gestern.
• Heute fühle ich mich in Mathe sicherer als gestern.
• Ich fühle mich heute in Mathe sicherer als gestern.

All of these are grammatical. The difference is mainly emphasis:

• In Mathe ... emphasizes the subject area.
• Heute ... emphasizes the time.
• Ich ... is the more neutral subject-first version.

Sometimes, but the meaning changes a bit.

• sicher here means sure/confident
• selbstsicher means self-confident or sometimes self-assured

In a math context, sicherer is usually the more natural choice:

• Ich fühle mich in Mathe sicherer.

Selbstsicherer sounds more like personality or social confidence:

• Sie wirkt sehr selbstsicher.

So in this sentence, sicherer is the better and more idiomatic word.