Ce marteau est plus lourd que l’autre, mais il est plus solide.

Because marteau is a masculine singular noun, and it begins with a consonant.

French demonstratives work like this:

• ce
  • masculine singular noun starting with a consonant
• cet
  • masculine singular noun starting with a vowel or mute h
• cette
  • feminine singular noun
• ces
  • plural noun

So:

• ce marteau = this hammer
• cet outil = this tool
• cette table = this table
• ces marteaux = these hammers

This is the normal French pattern for saying more ... than with an adjective:

plus + adjective + que

So:

• plus lourd que = heavier than
• plus solide que = more solid / sturdier than

You can use the same structure with many adjectives:

• plus grand que = taller/bigger than
• plus rapide que = faster than
• plus cher que = more expensive than

Related comparison patterns:

• moins ... que = less ... than
• aussi ... que = as ... as

Because autre begins with a vowel sound, and French normally shortens le or la to l’ before a vowel.

So:

• le autre → l’autre
• la autre → l’autre

In this sentence, l’autre means the other one or the other hammer.

Because French often leaves out a noun when it is already understood from context.

So que l’autre naturally means:

• than the other one
• or more specifically here, than the other hammer

You could say que l’autre marteau, but in many situations it sounds less natural because the noun is already obvious.

Here il refers back to ce marteau.

So the sentence is talking about the same hammer twice:

• Ce marteau est plus lourd que l’autre
• mais il est plus solide

In other words, this hammer is both:

• heavier
• and more solid

Since marteau is masculine singular, the pronoun is il.

Yes. French adjectives usually agree with the noun they describe in gender and number.

Here, marteau is masculine singular, so we get:

• lourd
• solide

If the noun were feminine singular, you would usually change the adjective form:

• lourde for feminine singular
• solide stays the same in the singular masculine and feminine

Examples:

• Cette boîte est plus lourde.
• Cette table est plus solide.

In the plural:

• lourds / lourdes
• solides

So solide is one of those adjectives whose masculine and feminine singular forms look the same.

In normal French, yes.

French usually needs an explicit subject in a full clause, so:

• ..., mais il est plus solide. = normal
• ..., mais est plus solide. = not standard

English sometimes allows more omission in certain styles, but French is stricter here.

In comparisons like this, plus is usually pronounced with a silent final s before a consonant.

So:

• plus lourd ≈ plu lourd
• plus solide ≈ plu solide

If plus comes before a vowel sound, you may hear a z sound because of liaison:

• plus utile → plu-z-utile

For a learner, the safest starting point here is:

• in plus lourd
• and plus solide

the final s is normally not pronounced.

Grammatically, solide does mean solid, but in English, with a tool like a hammer, a more natural translation is often:

• sturdier
• stronger
• more robust

So the exact English wording may change depending on style, even though the French word is still solide.

Yes, depending on the situation.

French can be more precise when contrasting two objects:

• Ce marteau-ci est plus lourd que celui-là, mais il est plus solide.

This sounds like:

• This hammer here is heavier than that one, but it is sturdier.

The original sentence, though, is completely natural when both hammers are already clear from context.

Because after a comparative adjective like plus lourd, French uses que directly.

So the pattern is:

• plus + adjective + que + noun/pronoun

Examples:

• plus lourd que l’autre
• plus grand que moi
• plus intéressant que ce livre

You do not use de after this kind of comparison.