Let's continue our series on the use of the verbs ser and estar, now focusing on how you can use soy (“I'm”—the first-person singular form of ser in the present tense) to talk about yourself.
 

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The present tense of the verb ser (to be) is soy. You can use it combined with an adjective (or a participio—the -ado, -ido, -to, -so, -cho endings and their feminine and plural forms, used as an adjective) to express an intrinsic characteristic or status, a permanent state of mind, body, or soul.
 
For starters, you can use it to introduce yourself:

Soy Paco, de 75 Minutos. -Hola.

I'm Paco, from 75 Minutes. -Hello.

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You can also use soy to talk about your occupation, career, etc.
 

Yo soy guardia civil.

I am a Civil Guard.

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And you can use soy to talk about your personality, preferences, nationality, beliefs or affiliations. For example: Yo soy musulmán (I'm muslim), soy miembro del partido (|'m a member of the party), soy tu hada madrina (I'm your fairy godmother).
 

Soy buena clienta, sí. La verdad que sí.

I am a good customer, yes. I truly am.

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Yo soy bastante escrupulosa y no me da nada.

I am pretty fussy and it doesn't bother me at all.

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The verb soy can also be used to talk about a role, status, function, etc:
 

Tú eres testigo. -Yo soy testigo. -Tú eres testigo.

You're a witness. -I'm a witness. -You're a witness.

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We mentioned, in our previous lesson on the subject, that estoy can also be used to talk about roles when combined with the preposition de, so saying yo estoy de testigo is also correct. There are subtle differences, though, which sometimes get lost in translation:
 
Yo soy testigo - I'm a witness
Yo estoy de testigo - I'm (working as) a witness
 
It's perhaps at this point, when these verbs are combined with adjectives (or participios used as adjectives), that English speakers get the most confused about the difference between soyand estoy. It gets even more confusing because in many cases it may seem Spanish speakers use both verbs indistinctly. Here are some examples:
 
Yo soy casado - I'm (a) married (person).
Yo estoy casado - I'm married.
Yo soy gordo - I'm (a) fat (person).
Yo estoy gordo - I'm fat.
Yo soy pequeña - I'm (a) small (person).
Yo estoy pequeña - I'm small.
 
Sometimes, however, it's impossible to use them indistinctly. It happens more frequently when the verbs are combined with participios (-ado, -ido, -to, -so, -cho endings), which take estar much more easily than ser:
 
Yo estoy devastado - I'm devastated.
*Yo soy devastado - Incorrect, don't use it.
Yo estoy cansado - I'm tired.
*Yo soy cansado - Incorrect, don't use it.
Yo estoy herido - I'm wounded.
*Yo soy herido - Incorrect, don't use it.
Yo estoy muerto - I'm dead.
*Yo soy muerto - Incorrect, don't use it.
 
*It's interesting how this may be different while using other modes or tenses. For example both yo estuve herido and yo fui herido (I was wounded) are possible, given the right context. However, fui herido is actually far more common than yo estuve herido, which would need a special context to make proper sense, for example: Yo estuve herido sin recibir ayuda por 10 horas (I was wounded without receiving any help for 10 hours).
 
The verb soy is also frequently combined with prepositions. For example, when combined with the preposition de, the verb soy indicates origin. So, besides soy mexicano (I'm Mexican) you can also say soy de México (I'm from Mexico).
 
Typically, the verb soy is followed by articles, but estoy doesn't take articles. Compare these:
 
Soy el mejor (I'm the best), soy mejor (I'm better), and estoy mejor (I feel better) are correct, but never say estoy el mejor. 
Soy tu padre (I'm your father), soy padre (I'm a father / also "I'm a nice person") and even estoy padre (I feel or look good) are correct, but you can't say estoy el padre.
Soy buena (I'm good), soy la buena (I'm the good one), estoy buena (I'm hot, good looking) are correct, but never say estoy la buena.
 

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The same happens with pronouns. You won't find a pronoun naturally following the verb estar, except, maybe, when you want to reiterate the subject and change the natural order of words (hyperbaton) for emphatic or stylistic purposes: estoy yo tan triste (me, I feel so sad). Normally, you'd say estoy tan triste (I feel so sad). This could also be done with ser: soy yo tan triste (me, I'm such a sad person).  But again, normally you'd just say soy tan triste (I'm such a sad person).
 
There are many other ways in which you can use the verb soy; these are just some of the most common ones.