Si je ne connais pas un mot, je cherche sa définition dans le dictionnaire.

No. In this sentence, si means if, not yes.

French si has two common uses:

• if: Si je ne connais pas un mot... = If I don't know a word...
• yes in response to a negative question or statement:
  Tu n'as pas faim ? — Si ! = Aren't you hungry? — Yes, I am!

So here, si is introducing a condition.

Standard French uses ne ... pas to make a sentence negative:

• je connais = I know
• je ne connais pas = I do not know / I don't know

In spoken informal French, people often drop ne:

• je connais pas

But in careful, written, or standard French, ne ... pas is the expected form.

This is one of the most important distinctions in French.

French often uses:

• savoir for knowing a fact, knowing how, or being aware of something
• connaître for being familiar with a person, place, or thing

A word is treated as something you are familiar with or not familiar with, so French says:

• connaître un mot = to know a word

You would not normally say savoir un mot here.

Compare:

• Je connais ce mot. = I know this word.
• Je sais ce que ce mot veut dire. = I know what this word means.
• Je sais parler français. = I know how to speak French.

Because the subject is je, so the verb must match je in the present tense.

The verb connaître in the present tense goes like this:

• je connais
• tu connais
• il/elle connaît
• nous connaissons
• vous connaissez
• ils/elles connaissent

So je ne connais pas is the correct form for I don't know.

Because this sentence expresses a general habit or general truth:

• Si je ne connais pas un mot, je cherche sa définition...
• If I don't know a word, I look up its definition...

French often uses the present tense in both parts of a si sentence when talking about something habitual, repeated, or generally true.

It is not talking about one specific future event, but about what the speaker usually does.

The comma separates the if-clause from the main clause:

• Si je ne connais pas un mot, = If I don't know a word,
• je cherche sa définition dans le dictionnaire. = I look up its definition in the dictionary.

This is very natural in both French and English when the conditional clause comes first.

Because each clause needs its own subject and verb.

French says:

• Si je ne connais pas un mot, je cherche...

Both clauses are complete:

1. je ne connais pas
2. je cherche

English also does this:

• If I don't know a word, I look up...

So the second je is necessary.

Un mot means a word in a general sense, not a specific word already identified.

The sentence means:

• whenever I don't know a word / any word

If you said le mot, it would usually refer to a specific word already known from context:

• Si je ne connais pas le mot... = If I don't know the word...

So un mot is the natural choice for a general statement.

Sa means his, her, or its, depending on what noun it refers to.

Here it refers to un mot:

• un mot = a word
• sa définition = its definition

So the meaning is:

• I look for its definition

In French, possessive adjectives agree with the thing possessed, not with the owner:

• définition is feminine
• so French uses sa

That is why it is sa définition, even though mot is masculine.

Because son/sa/ses agree with the noun that follows, not with the noun doing the possessing.

Here, the possessed thing is définition, which is feminine:

• la définition
• so: sa définition

Compare:

• un mot et sa définition = a word and its definition
• un mot et son genre = a word and its gender

Why?

• définition is feminine → sa
• genre is masculine → son

Literally, chercher means to look for or to search for.

So:

• je cherche sa définition = I look for its definition

In natural English, when using a dictionary, we often say look up:

• I look up its definition in the dictionary

So the French verb is literally look for, but the most natural English translation in this context is often look up.

Dans le dictionnaire literally means in the dictionary, which is the normal way to say that the definition is found inside that source.

• dans = in / inside

So:

• je cherche sa définition dans le dictionnaire = I look for its definition in the dictionary

Au dictionnaire would not be the normal phrasing here.

Le dictionnaire means the dictionary.

French often uses the definite article when speaking about something understood in a general or practical sense:

• dans le dictionnaire = in the dictionary

In English, we might sometimes say in a dictionary if we mean any dictionary at all. French can certainly say dans un dictionnaire too, but dans le dictionnaire is very natural when referring to the dictionary as the normal reference tool.

So:

• dans le dictionnaire = in the dictionary
• dans un dictionnaire = in a dictionary

Both are possible, but le dictionnaire sounds very natural in this kind of general statement.

It most naturally describes a habit or general behavior.

The structure:

• Si + present, present

often expresses:

• what someone usually does
• a general truth
• a routine

So the idea is:

• Whenever I don't know a word, I look up its definition in the dictionary.

If you wanted to describe one specific future situation, French might use a different tense pattern depending on the meaning.

A very natural English version would be:

• If I don't know a word, I look it up in the dictionary.

That is slightly more idiomatic than translating every word directly.

The French sentence explicitly says its definition, but English often shortens that idea to look it up. Both express the same basic meaning here.