J’aurais dû ouvrir le dictionnaire plus tôt, mais j’ai voulu deviner le sens toute seule.

Yes. J’aurais dû is the natural French way to say I should have in this context.

It is built like this:

• j’aurais = I would have
• dû = past participle of devoir (must / have to / should)

So:

• J’aurais dû ouvrir le dictionnaire = I should have opened the dictionary

This structure often expresses regret or criticism about the past.

Compare:

• Je dois ouvrir le dictionnaire = I must / have to open the dictionary
• Je devais ouvrir le dictionnaire = I was supposed to open the dictionary / had to open it
• J’ai dû ouvrir le dictionnaire = I had to open the dictionary
• J’aurais dû ouvrir le dictionnaire = I should have opened the dictionary

Because je becomes j’ before a word that begins with a vowel sound.

So:

• je aurais → j’aurais

This is very common in French:

• j’ai
• j’aime
• j’ouvre
• j’habite

It helps pronunciation flow more smoothly.

After devoir in this kind of structure, French uses the infinitive of the main verb.

So:

• j’aurais dû ouvrir = I should have opened
• not j’aurais dû ouvrais or j’aurais dû ai ouvert

This is the same pattern you see with many modal-like verbs in French:

• je veux partir = I want to leave
• je peux venir = I can come
• je dois travailler = I must work

Here, ouvrir stays in the infinitive because devoir is the conjugated verb.

This is a very important difference.

• J’ai dû ouvrir le dictionnaire = I had to open the dictionary
• J’aurais dû ouvrir le dictionnaire = I should have opened the dictionary

So:

• j’ai dû expresses necessity in the past
• j’aurais dû expresses what would have been the better thing to do

In this sentence, the speaker is looking back and saying they made the wrong choice. That is why j’aurais dû is used.

Yes, literally it is more early, but in natural English it means earlier.

So:

• plus tôt = earlier / sooner
• plus tard = later

In the sentence:

• ouvrir le dictionnaire plus tôt = to open the dictionary earlier

French often forms comparatives this way:

• plus grand = bigger / taller
• plus vite = faster
• plus tôt = earlier

Because j’ai voulu presents the action as a completed choice or decision in the past:

• mais j’ai voulu deviner le sens = but I wanted to guess the meaning

Here, the speaker is referring to a specific moment or decision: instead of opening the dictionary, they chose to try guessing.

By contrast:

• je voulais often means I wanted to in a more ongoing, background, or habitual sense

Compare:

• À ce moment-là, j’ai voulu essayer seule. = At that moment, I wanted to try on my own.
• Quand j’étais petite, je voulais tout faire seule. = When I was little, I wanted to do everything by myself.

So j’ai voulu fits the idea of a specific past decision.

Deviner means to guess.

So:

• deviner le sens = to guess the meaning

Here, le sens means the meaning of a word, expression, or passage.

A learner might also see:

• deviner un mot = to guess a word
• deviner la réponse = to guess the answer
• deviner le sens d’un mot = to guess the meaning of a word

So in this sentence, the speaker tried to work out the meaning without checking the dictionary.

Both are possible, but toute seule adds emphasis.

• seule = alone / by herself
• toute seule = all alone / all by herself / entirely on her own

In this sentence, toute seule suggests:

• without anyone’s help
• on my own
• possibly with a tone like I insisted on doing it myself

So:

• j’ai voulu deviner le sens seule = possible, but less emphatic
• j’ai voulu deviner le sens toute seule = more natural and expressive here

Because it refers to the speaker, and the sentence shows that the speaker is female.

The base form is:

• masculine: tout seul
• feminine: toute seule

So:

• a man would say: j’ai voulu deviner le sens tout seul
• a woman says: j’ai voulu deviner le sens toute seule

This expression agrees with the person it describes.

The circumflex accent in dû helps distinguish it from du.

• dû = past participle of devoir
• du = contraction of de + le

Compare:

• J’ai dû partir. = I had to leave.
• Je viens du marché. = I’m coming from the market.

In modern French, accents like this sometimes help avoid confusion between words that would otherwise look identical.

French often uses the definite article where English might use a, the, or sometimes no article, depending on context.

Here, ouvrir le dictionnaire sounds very natural because it refers to the dictionary as the obvious tool to use in that situation.

French often does this with general, expected, or contextually understood things:

• prendre le train = to take the train
• jouer du piano = to play the piano
• ouvrir le dictionnaire = to open the dictionary

English and French do not always match exactly in article usage, so it is best to learn these as natural French patterns.

Because they express two different ideas about the past.

• j’aurais dû ouvrir le dictionnaire plus tôt = a reflection or regret about what would have been better
• j’ai voulu deviner le sens toute seule = the actual action or decision that happened

So the sentence contrasts:

1. what the speaker now thinks was the right thing to do
2. what the speaker actually chose to do at the time

That contrast is exactly why the sentence works so well:

• I should have opened the dictionary earlier
• but I wanted to guess the meaning by myself

The grammar supports the meaning very clearly.