You should not be confused by (at ) the plural form in an adjective phrase or in an adjective clause. Just ignore the adjective phrase or the adjective clause. Then find the subject of the sentence. You can find a suitable verb form in the blank. By changing the base verb 'be' into a suitable verb form, you should fill in the blank. You should use the present tense verb her, though you can use the past tense form.

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In the firse line, which preposition is correct? Can we use both prepositions?
Would you check all the sentences above?
Thank you.

You should not be confused by a plural form in an adjective phrase or clause. Just ignore the adjective phrase or clause. Then find the subject of the sentence. You can then use the correct verb form in the blank by changing the base verb 'be' to agree with the subject. You should use the present tense verb here although you can also use the past tense form.

Instead of "suitable" (which is not necessarily incorrect), we usually refer to making sure that subjects and verbs agree or to the agreement of subjects and verbs.
http://grammar.ccc.commnet.edu/grammar/sv_agr.htm

=)

In the first line, the correct preposition to use is "at." The phrase "confused by" indicates that "confused" is the feeling or state attributed to the subject of the sentence.

As for checking the sentences above, yes, I can definitely do that for you. Here's a breakdown of the analysis for each sentence:

1. "You should not be confused by the plural form in an adjective phrase or in an adjective clause."
- The subject of the sentence is "You."
- The verb form in the blank can be filled with "be." ("You should not be confused.")
- The adjective phrase "by the plural form in an adjective phrase or in an adjective clause" can be ignored for this analysis.

2. "Just ignore the adjective phrase or the adjective clause."
- The subject of the sentence is "You."
- The verb form in the blank can be filled with "ignore." ("Just ignore.")
- The phrase "the adjective phrase or the adjective clause" is the direct object of the verb "ignore."

3. "Then find the subject of the sentence."
- The subject of the sentence is "You."
- The verb form in the blank can be filled with "find." ("Then find.")
- There are no additional phrases to consider in this sentence.

4. "You can find a suitable verb form in the blank."
- The subject of the sentence is "You."
- The verb form in the blank can be filled with "find." ("You can find.")
- The phrase "a suitable verb form" is the direct object of the verb "find."

5. "By changing the base verb 'be' into a suitable verb form, you should fill in the blank."
- The subject of the sentence is "you."
- The verb form in the blank can be filled with "fill." ("You should fill.")
- The phrase "changing the base verb 'be' into a suitable verb form" is used as an adverbial phrase.

I hope this helps clarify the analysis of these sentences for you. If you have any further questions, feel free to ask!