Il significato di questa frase cambia molto se togli l’accento.

Because frase is a feminine noun in Italian, so the demonstrative has to agree with it:

• questa frase = this sentence/phrase
• questo libro = this book

Italian demonstratives and articles change for gender and number, so agreement matters much more visibly than in English.

Italian commonly uses di to link two nouns, where English often uses of:

• il significato di questa frase = the meaning of this sentence

So the structure is:

• il significato = the meaning
• di questa frase = of this sentence

This is the normal and natural way to say it in Italian.

Cambia means changes, and molto here means a lot or greatly.

So cambia molto means:

• changes a lot
• changes greatly
• changes significantly

It does not necessarily mean the meaning becomes completely different, just that the change is important.

Because here molto is an adverb, not an adjective.

It modifies the verb cambia:

• cambia molto = changes a lot

Adverbs do not agree in gender or number. Compare:

• molta acqua = a lot of water → adjective, so it agrees with acqua
• cambia molto = changes a lot → adverb, so it stays molto

Here se means if.

So:

• se togli l’accento = if you remove the accent

This is a very common structure in Italian for real or possible conditions:

• Se studi, impari. = If you study, you learn.
• Se togli l’accento, cambia il significato. = If you remove the accent, the meaning changes.

In this sentence, togli is present indicative, not an imperative.

Why?

Because after se meaning if, Italian normally uses the indicative in this kind of real condition:

• se togli = if you remove

It happens that the tu present form and the tu imperative form of togliere are the same:

• tu togli = you remove
• togli! = remove!

So the form looks identical, but the context tells you which one it is. After se, it is understood as if you remove.

Because Italian usually leaves out subject pronouns when the verb ending already makes the subject clear.

• togli already tells us the subject is tu
• so se togli l’accento is perfectly natural

You could say se tu togli l’accento, but that would usually add emphasis or contrast.

Because the definite article is shortened before a vowel.

• il or lo becomes l’ before many singular nouns beginning with a vowel
• accento begins with a
• so we get l’accento

Also, the mark in l’accento is an apostrophe, not the accent being talked about. English speakers often mix those up at first.

Here it almost certainly means a written accent mark on a vowel, such as in:

• è
• sé
• là
• sì

Italian accento can refer to stress or accent in other contexts, but in a sentence like this, it normally means the written mark. The idea is that removing that mark can change the meaning of the sentence a lot.

Yes. Italian word order is fairly flexible.

This version:

• Il significato di questa frase cambia molto se togli l’accento.

could also be expressed as:

• Se togli l’accento, il significato di questa frase cambia molto.

Both are natural. The original puts the main statement first, while the second version puts the condition first.

No. You normally pronounce it smoothly as one connected phrase:

• l’accento

The apostrophe just shows that the article has been shortened. It does not create a strong pause.