El borde y la base tienen el mismo grosor.

Because the subject is plural: el borde y la base = the edge and the base.

In Spanish, when two nouns are joined by y (and), the verb normally goes in the plural.

• El borde tiene... = The edge has...
• La base tiene... = The base has...
• El borde y la base tienen... = The edge and the base have...

So tienen is the correct they have form.

Because borde is a masculine noun and base is a feminine noun.

In Spanish, nouns have grammatical gender, so the article has to match:

• el borde
• la base

This is something you often just have to learn with each noun. The ending does not always make it obvious:

• base ends in -e but is feminine
• borde also ends in -e but is masculine

So it is best to learn them together with their articles: el borde, la base.

Because mismo agrees with grosor, not with base or borde.

The noun here is grosor (thickness), and grosor is masculine:

• el grosor
• therefore: el mismo grosor

Even though base is feminine, that does not matter here, because mismo is describing grosor.

Compare:

• la misma base = the same base
• el mismo grosor = the same thickness

Here mismo means same.

So el mismo grosor means the same thickness.

This is a very common pattern in Spanish:

• el mismo color = the same color
• la misma forma = the same shape
• el mismo tamaño = the same size

In this sentence, it tells us that the edge and the base are equal in thickness.

Because grosor is a noun, while grueso is usually an adjective.

• grosor = thickness
• grueso / gruesa = thick

So:

• tienen el mismo grosor = they have the same thickness
• son igual de gruesos/gruesas = they are equally thick

Both can be correct depending on what you want to say, but this sentence is built around the noun grosor.

A useful comparison:

• La pared es gruesa. = The wall is thick.
• La pared tiene mucho grosor. = The wall has a lot of thickness / is very thick.

Yes, in many contexts you could.

• El borde y la base tienen el mismo grosor.
• El borde y la base son del mismo grosor.

Both mean roughly the same thing: the edge and the base are the same thickness.

That said, tienen el mismo grosor is often the more straightforward and neutral way to say it, especially in technical or descriptive contexts.

In Spanish, mismo often appears with the definite article in expressions like el mismo / la misma / los mismos / las mismas when you mean the same.

So Spanish naturally says:

• el mismo grosor = the same thickness
• la misma altura = the same height
• el mismo tamaño = the same size

Leaving out the article here would sound incomplete or unnatural in standard Spanish.

Because the sentence is talking about one shared property: thickness.

The meaning is not that each thing has different thicknesses, but that both of them share the same thickness. So Spanish uses the singular:

• el mismo grosor

A plural like los mismos grosores would suggest multiple thickness values, which is not what this sentence means.

Yes. Borde can mean edge, rim, or border, depending on the context.

For example:

• the edge of a table
• the rim of a glass
• the border of a shape

In Spain, there is also an informal meaning where borde can describe a person as rude or unfriendly, but that is completely unrelated here. In this sentence, it is clearly the physical edge.

It can mean several related things, depending on context:

• base = base
• the bottom part of something
• a foundation or support
• the base in a geometric or technical sense

In this sentence, it most likely refers to the lower or supporting part of an object.

Yes, it is completely natural and standard in Spanish from Spain.

It sounds especially natural in:

• technical descriptions
• product descriptions
• geometry
• design or manufacturing contexts

It is a clear, idiomatic way to say that the edge and the base have equal thickness.