Learn English – how to use the verb “marry” in passive form

There is no difference in literal meaning. Sit down is the more common phrasing. You would use Sit yourself down only under certain circumstances:

1) Regional accent. This formation is characteristic of certain regional accents or dialects.

2) Informality. The use of "yourself" would rarely be used in a formal setting; it might be used to convey informality: "Why don't you sit yourself down and stay a spell."

3) Emphasis. It can also be used to add emphasis to a command. "Bobby, you sit yourself down right now and stop that foolish talk."

The verb which follows a modal verb (can/could, may/might, must, shall/should, will/would) in a verb group is always a ‘bare’ infinitive—that is, an infinitive which is not ‘marked’ with to. (The presence of not or an adverb does not affect this rule.) Marked infinitives and partiples cannot occur in this position.

^okShe cannot tolerate^{bare infinitive} his rudeness.
^∗ She cannot to tolerate^{marked infinitive} his rudeness.
^∗ She cannot tolerating^{present participle} his rudeness.
^∗ She cannot tolerated^{past participle} his rudeness.
^∗ _{marks a form as ungrammatical}

However, modal verbs can combine with progressive, perfect and passive constructions. When that is the case, it is only the first auxiliary after the modal which is cast in the infinitive. Subsequent auxiliaries and the main or ‘lexical’ verb take the form required by the construction. (But none of these constructions takes a marked infinitive).

PROGRESSIVE: ^okShe cannot be^{bare infinitive} tolerating^{present participle} his rudeness.
PERFECT: ^okShe cannot have^{bare infinitive} tolerated^{past participle} his rudeness.
PASSIVE: ^okHis rudeness cannot be^{bare infinitive} tolerated^{past participle} by her.
PERFECT PASSIVE: ^okHis rudeness cannot have^{bare infinitive} been^{past participle} tolerated^{past participle} by her.