Effect of adding an Independent Variable in Multiple Linear Regression I am new in machine learning and learning linear regression concept. Please help with answers to below queries.
I want to understand effect on existing independent variable(X1) if I add a new independent variable(X2) in my model.
This new variable is highly correlated with dependent variable(Y)

Will it have any effect on beta coefficient of X1?
Will relationship between X1 and Y become insignificant?
Can adjusted R-square value decrease?
Yes, the addition of the independent variable ($X_2$) will have an impact on the beta coefficient of $X_1$.
Maybe, it depends on the relationship between $X_1$ and $Y$, and whether you are using any regularization or not.
If the independent variable $X_2$ is more correlated than $X_1$, then the value of $r^2$ should be higher (but I am not sure on this)