The concept of subject-verb agreement of Spanish is similar to that of English: because the subject determines the number and person of a sentence, the verb in the predicate must agree in number and person with the subject. Consider the following example.
[Yo] [soy bilingüe.]
[I] [am bilingual.]
Because the subject, yo, is singular first person, the verb must be conjugated in the same number and person. The verb soy is the singular first person of the present tense conjugation of the verb ser, so in this sentence, the subject and verb agree. If you want to double-check the conjugation of a verb in simple tenses or if you do not have experience (or the confidence) conjugating verbs in Spanish, you can search for the correct conjugation on the website (and the smartphone app) of the Real Academia Española (RAE).
On RAE’s home page, select the Diccionario de la lengua española from the drop-down menu on the top left of the page. Then type in any form or tense of a verb (in this case ser) on the search box located to the right of the drop-down menu.
The page with the definition of ser will open in a new tab. On this new page, click on the Conjugar button on the left side of the page to open the conjugation table.
On the table, verbs are classified as either personal or impersonal.
The infinitive (infinitivo), gerund (gerundio), and participle (participio) are considered impersonal forms because they do not express person and number. The simple and compound personal forms do express person and number; therefore, they can be used in different tenses of the indicative, subjunctive, and imperative moods. To get better acquainted with the RAE website, please focus on the indicative mood on the following table.
Note that the table allows for the selection of the tense you need to use; soy is the singular first person of the present tense of the indicative mood. You can use this conjugation tool to double-check that the verb agrees in person and number with the subject of the sentence you are writing.
This tool, however, does not provide compound tenses, which RAE includes as part of the conjugation paradigm. In sections 23.1h and 23.1i, Nueva gramática offers a few arguments as to why compound tenses are included in the regular conjugation paradigm; its aspectual property of perfectivity is the strongest one. To fully express the completeness of an event, i.e., its perfectivity, compound tenses are necessary. Compound tenses are made by conjugating the auxiliary verb haber and adding the participle of the main verb (1676-1677). I will discuss these tenses in a future article.
FYI: The particularities of subject-verb agreement need to be considered to make sure that the verb agrees with its subject.