Access Modifiers with Interfaces

Since C# 8.0 you can have access modifier inside the interface. See this post C# 8 Interfaces: Public, Private, and Protected Members

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Before C# 8.0

You should see:

Access Modifier - MSDN

Interfaces declared directly within a namespace can be declared as public or internal and, just like classes and structs, interfaces default to internal access. Interface members are always public because the purpose of an interface is to enable other types to access a class or struct. No access modifiers can be applied to interface members.

(For your questions)

Can we use Access Modifier with Interface

Yes, they can be declared as public or internal

By default Are inteface internal

Yes.

what about Access Modfiers with Inteface members.

They are public. No access modifiers can be applied to interface members.