"Ele sunt subțiri."

Translation: They are thin.

[mckara]

Why masculine subțiri after feminine ele?

[DeeaSab]

Its called declinarea adjectivului. Some adjectives have 2 forms at singular and 2 at plural(deștept/deșteaptă-deștepți/deștepte=smart), some 2 at singural and 1 at plural(prevăzător/prevăzătoare-prevăzători=cautious), 1 singural and 1 plural(veche-vechi=old). They are also adjectives with only one form(maro does not have a plural form=brown)

[MariaMyshk]

what is the difference between slabi and subtiri? can you make some examples?

[DeeaSab]

Eu sunt slab=I am weak or skinny.
Lemnul este subțire=The wood is thin.
Slab is used for people, subțire for objects, but if you use subțire for people(I am subțire) you accentuate how thin someone is.

[bluchill]

Skinny should work, it was there in their previous rendition of the answer.