Quand Marie voit une fissure près de la fenêtre, elle se met tout de suite à chercher une échelle.

Quand means when.

Here it introduces a time clause: Quand Marie voit une fissure... = When Marie sees a crack...

In French, quand can be used:

• as a question word: Quand arrives-tu ? = When are you arriving?
• as a conjunction introducing a clause: Quand il pleut, je reste chez moi. = When it rains, I stay home.

In your sentence, it is the second use.

Voit is the 3rd person singular present form of voir (to see), because the subject is Marie.

Present tense of voir:

• je vois
• tu vois
• il/elle/on voit
• nous voyons
• vous voyez
• ils/elles voient

So:

• Marie voit = Marie sees
• je vois = I see

It is not voit-elle because this is not a question. It is a normal statement.

French often uses the present tense in time clauses introduced by words like quand when talking about something that happens generally or as part of a sequence.

So:

• Quand Marie voit une fissure, elle... = When Marie sees a crack, she...

This can sound like:

• a habitual action
• a narrative in the present
• a general sequence of events

French does not need a future tense here.

Compare:

• Quand il pleut, je prends un parapluie. = When it rains, I take an umbrella.

French usually requires an article where English may sometimes omit one.

Une fissure means:

• a crack
• one crack
• some crack in context

Because fissure is a countable singular noun, French normally needs an article:

• une fissure = a crack
• la fissure = the crack

You generally cannot just say voit fissure in standard French.

Because une is the indefinite article, meaning a/an.

It is used when the thing is:

• being mentioned for the first time
• not specific to the listener
• one example of something

So:

• une fissure = a crack
• la fissure = the crack

Since the crack has not been identified before, une is the natural choice.

Près de means near or close to.

So:

• près de la fenêtre = near the window

Breakdown:

• près de = near
• la fenêtre = the window

Note that près is normally followed by de:

• près de la maison = near the house
• près du jardin = near the garden

Because the expression is près de.

So the structure is:

• près de + noun

Examples:

• près de la fenêtre = near the window
• près du lit = near the bed
• près des arbres = near the trees

You need the de because it belongs to the fixed expression près de.

Se mettre à + infinitive means to start/begin to do something.

So:

• elle se met à chercher = she starts looking
• more literally: she sets herself to looking

This is different from simple elle cherche, which just means:

• she is looking
• she looks for

The phrase se mettre à adds the idea of the beginning of the action.

Compare:

• Elle cherche une échelle. = She is looking for a ladder.
• Elle se met à chercher une échelle. = She starts looking for a ladder.

The verb here is se mettre à, a pronominal expression.

In this construction:

• je me mets à
• tu te mets à
• il/elle se met à
• nous nous mettons à
• vous vous mettez à
• ils/elles se mettent à

The se is not translated word-for-word in natural English here. It is just part of the expression se mettre à + infinitive, meaning to begin/start to.

So:

• elle se met à chercher = she starts looking

You should learn se mettre à as a whole expression.

Because the expression is se mettre à + infinitive.

So the pattern is:

• se mettre à faire quelque chose = to start doing something

Examples:

• Il se met à rire. = He starts laughing.
• Nous nous mettons à travailler. = We start working.
• Elle se met à chercher une échelle. = She starts looking for a ladder.

The à is required by this construction.

Tout de suite means right away, immediately, or straight away.

In this sentence:

• elle se met tout de suite à chercher...
• literally: she starts right away to look for...
• natural English: she immediately starts looking for...

It often appears after the conjugated verb or verb phrase, but adverb placement in French can be flexible.

Examples:

• Je viens tout de suite. = I’m coming right away.
• Il a tout de suite compris. = He understood immediately.

Because chercher directly means to look for. It does not use pour in this sense.

So:

• chercher une échelle = to look for a ladder

Not:

• chercher pour une échelle ❌

This is a common difference from English. In French, many verbs take their object directly.

Other examples:

• chercher ses clés = to look for one’s keys
• chercher un médecin = to look for a doctor

Échelle is a feminine noun, so it takes une in the singular indefinite form.

• une échelle = a ladder
• l’échelle = the ladder

It begins with a vowel, so in the definite form you get l’échelle, not la échelle.

Also, in your sentence it is indefinite because it means a ladder, not a specific previously mentioned ladder.

French, like English, often uses a pronoun to avoid repeating the same noun.

So:

• Quand Marie voit une fissure..., elle...
• When Marie sees a crack..., she...

Elle refers back to Marie.

French could repeat Marie, but it would sound heavier:

• Quand Marie voit une fissure..., Marie se met...

Using elle is the natural choice.

It could be understood in more than one way depending on context.

With the present tense, this sentence can sound like:

1. a habitual action
   
   • Whenever Marie sees a crack near the window, she immediately starts looking for a ladder.
2. a narrative present
   
   • In a story being told vividly in the present: Marie sees a crack, then immediately starts looking for a ladder.
3. a general sequence of actions
   
   • One action triggers the next.

Without more context, all of these are possible.

A very literal breakdown would be:

• Quand = when
• Marie = Marie
• voit = sees
• une fissure = a crack
• près de la fenêtre = near the window
• elle = she
• se met = sets herself / starts
• tout de suite = right away
• à chercher = to look for
• une échelle = a ladder

So a near-literal version would be:

When Marie sees a crack near the window, she starts right away to look for a ladder.

A more natural English translation is:

When Marie sees a crack near the window, she immediately starts looking for a ladder.