Ce modèle de chaise irait bien sur la terrasse, mais celui-là prend trop de place dans l’allée.

Because ce agrees with modèle, not with chaise.

• modèle is masculine singular, so you use ce
• chaise is feminine, but it comes later in the phrase as part of de chaise

So the structure is:

• ce modèle = this model
• de chaise = of chair / chair type

If you were talking directly about the chair itself, you would say cette chaise.

Because de chaise is being used in a general, classifying sense.

Here, modèle de chaise means something like:

• chair model
• type of chair
• chair design

French often uses de + noun with no article when the second noun simply identifies the category.

Compare:

• un modèle de chaise = a chair model / a type of chair
• le modèle d’une chaise = the model/design of a particular chair

So in your sentence, it is general, not about one specific chair.

Irait bien is the conditional of aller bien.

Literally, aller bien means to go well, but in this kind of sentence it often means:

• would look good
• would work well
• would fit nicely
• would suit the space

So Ce modèle de chaise irait bien sur la terrasse means something like:

• This model of chair would look good on the terrace
• This chair model would work well on the terrace

Because the speaker is making a suggestion or evaluation, not stating a simple fact.

• va bien = goes well / works well
• irait bien = would go well / would work well

The conditional irait makes the statement sound:

• more hypothetical
• more tentative
• more polite
• more like design advice or an opinion

So it has the feeling of:

• This would be a good choice for the terrace

rather than

• This definitely goes well on the terrace

Yes. Irait comes from aller in the conditional present.

The verb aller is irregular. Its conditional stem is ir-.

So:

• j’irais
• tu irais
• il / elle / on irait
• nous irions
• vous iriez
• ils / elles iraient

In your sentence, the subject is ce modèle, which is singular, so you get irait.

Because sur is the normal preposition here for being on / out on a terrace.

In French, sur is commonly used for spaces that are treated a bit like surfaces or open areas:

• sur la table
• sur le balcon
• sur la terrasse

So sur la terrasse is the natural way to say on the terrace or out on the patio/terrace.

Celui-là is a demonstrative pronoun. It means that one.

It is used to replace a masculine singular noun that has already been mentioned or is understood from context. Here it replaces ce modèle de chaise.

So instead of repeating the full noun phrase, French says:

• ce modèle de chaise ... celui-là

That is like saying in English:

• this chair model ... that one

It helps avoid repetition.

Because it refers back to modèle, which is:

• masculine
• singular

So the correct demonstrative pronoun is celui.

Quick comparison:

• celui-là = that one masculine singular
• celle-là = that one feminine singular
• ceux-là = those ones masculine plural
• celles-là = those ones feminine plural

Even though chaise is feminine, the pronoun still matches modèle, because that is the noun being replaced.

Traditionally:

• celui-ci = this one
• celui-là = that one

But in real modern French, -là is very often used simply to point out or contrast one item, even when English might not strongly emphasize distance.

In this sentence, celui-là is natural because the speaker is contrasting two chair models:

• one model would work well on the terrace
• that one takes up too much room in the walkway

So celui-là helps mark the contrasting item.

Because -ci and -là are attached to demonstratives with a hyphen.

So you write:

• celui-ci
• celui-là
• ce livre-ci
• cette chaise-là

That hyphen is standard spelling.

Because prendre de la place is a very common French expression meaning:

• to take up space
• to occupy too much room

When you add a quantity expression like trop, French uses de:

• trop de place
• beaucoup de place
• assez de place

So:

• prend trop de place = takes up too much space

This is the normal idiomatic phrasing.

After expressions of quantity such as trop, beaucoup, assez, peu, French normally uses de without the article.

So:

• beaucoup de temps
• trop de bruit
• assez de place

Even though by itself you can say:

• prendre de la place

once you insert trop, it becomes:

• prendre trop de place

That is standard French grammar.

Allée can mean different things depending on context, including:

• path
• walkway
• aisle
• driveway

In this sentence, since the speaker is talking about furniture placement near a terrace, dans l’allée most likely means something like:

• in the walkway
• in the path
• in the passageway

The idea is that this chair model would block movement because it takes up too much room there.

Because French commonly uses dans for being in an aisle, passage, or pathway-like space.

So:

• dans l’allée
• dans le couloir
• dans le passage

This corresponds well to English in the aisle or in the walkway.

Because French uses elision before a vowel sound.

• la allée becomes l’allée

This happens very often:

• la école → l’école
• le arbre → l’arbre
• je aime → j’aime

So l’allée is simply the normal shortened form of la allée.

Yes, but it would sound heavier and less natural.

For example, they could say:

• Ce modèle de chaise irait bien sur la terrasse, mais ce modèle de chaise prend trop de place...

That repeats too much. French, like English, often prefers a pronoun once the noun is already clear.

So celui-là is the natural, elegant choice.