Only if can be used to connect the main clauseA group of related words containing a subject and a verb. and the if-clause to express a more restrictive condition.

Acetaminophen is dangerous to kids only if dosage is too high.

It is possible in all types of conditionals. Note that if the sentence starts with the condition, the main clauseA clause that could be a sentence in itself. E.g.: I wouldn’t go there if I were you. ("I wouldn't go there") When I arrived he was surfing. ("he was surfing") is invertA structure in which a verb or an auxiliary precedes the subject. E.g.: "Here comes the sun."ed.

Normally, the auxiliariesA helping verb used with main verbs to express mood, voice and tense. E.g.: I can't stand when I am disturbed. ("can", "am") of permission, possibility and willingness are used to express requests.

When telling a story in the past timeA concept which is related to our perception of reality; that part of existence which is measured in seconds, minutes, hours, days, weeks, etc., if an eventA general term expressing that somebody or something does something or that something happens. E.g.: An apple fell on Isaac’s head. quickly follows another, these adverbA part of speech that modifies verbs, adjectives, adverbs and conjunctions. E.g.: I could hardly utter a word. ("hardly")ials and partial inversion is used to express the first event. In the invertA structure in which a verb or an auxiliary precedes the subject. E.g.: "Here comes the sun."ed clauseA group of related words containing a subject and a verb. usually past perfectThe aspect expressing completion. E.g.: He’s finished university. is used. With no sooner … than, however, we can also use simple past.

To invertA structure in which a verb or an auxiliary precedes the subject. E.g.: "Here comes the sun." something means to put something in the opposite position to the one it was in before. In the case of English grammar this refers to the change of the word order of the sentence, when the verbA part of speech that expresses an action or a state. E.g.: John seldom plays tennis. or an auxiliaryA helping verb used with main verbs to express mood, voice and tense. E.g.: I can't stand when I am disturbed. ("can", "am") comes before the subjectThe person or thing that performs the action of a verb or that is described by a verb. E.g.: Andy has never been to Brunei. ("Andy"). The function of inversion is usually emphasisSpecial importance or attention given to something. E.g.: All I wanna do is have some fun..

There are two types of inversion:

We use the past perfect tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses. to express that an eventA general term expressing that somebody or something does something or that something happens. E.g.: An apple fell on Isaac’s head. took place before another event in the past, that is, it had been completeA concept, related to the perfect aspect, expressing that an action has finished. E.g.: He’s finished university.d. This tense allows us to keep track of the order of the events if this is not clear otherwise.

If + subjectThe person or thing that performs the action of a verb or that is described by a verb. E.g.: Andy has never been to Brunei. ("Andy") + auxiliaryA helping verb used with main verbs to express mood, voice and tense. E.g.: I can't stand when I am disturbed. ("can", "am") can be replaced in formalNot belonging to colloquial or everyday speech; official. E.g.: I demand that Mr Smith call the person on whom the committee has decided. English, or for dramatic effect, by the inversionA structure in which a verb or an auxiliary precedes the subject. E.g.: "Here comes the sun." of the auxiliary and the subject, with the if omitted.