How do I use 『on alternate days.』
1.I clean the room every other day
2.I clean the room on alternate days
3.He and I clean the room every other day
4.He and I clean the room on alternate days

I think 1 and 2 have the same meanings, and they mean 『I clean the room every 48 hours』, right?
Then, sentence 3 means 『We clean the room together, every 48 hours』right?
And I don't know what sentence 4 means? Is it 『I clean the room on Mon, Wed, Fri, and he dose it on Tue, Thu, Sat. . . . .』?
Thank you for helping

Question

How do I use 『on alternate days.』
1.I clean the room every other day
2.I clean the room on alternate days
3.He and I clean the room every other day
4.He and I clean the room on alternate days

I think 1 and 2 have the same meanings, and they mean 『I clean the room every 48 hours』, right?
Then, sentence 3 means 『We clean the room together,  every 48 hours』right?
And I don't know what sentence 4 means? Is it 『I clean the room on Mon, Wed, Fri, and he dose it on Tue, Thu, Sat. . . . .』?
Thank you for helping

Answer

You are right to be confused.  Sentence 4 is ambiguous.  It could mean either "He and I clean the room every 48 hours" OR it could mean "I clean the room on Mon, Wed, Fri, and he cleans it on Tue, Thu, Sat."

One way to remove the ambiguity would be to write Sentence 4 as follows: "He and I take turns cleaning the room on alternate days."