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One grammar mistake corrected :) In unstressed endings, never write "о" after "ш, ж, ч, щ and ц". The rest is right.

And "если" cannot be omitted in your introductory phrase.

Cumulus is absolutely right.

And your examples, Antonio, are right as well.

If there is a compound sentence like

Я видел девушку, которая читала книгу. - "девушка" is in accusative (green)...