Do stem changing verbs only effect first second and third person singular and third person plural when conjugating verbs in spanish?

Answer 1

In the Present Indicative (which is likely the tense you are thinking of), the answer is Yes. One example from each category (ar/er/ir):

Pensar: pienso, piensas, piensa, pensamos, pensáis, piensan

Encender: enciendo, enciendes, enciende, encendemos, encendéis, encienden

Servir: siervo, sierves, sierve, servimos, servís, sierven

However, once we move into other tenses and moods, there are more complex rules.

In the case of "ar" and "er" radical-changing verbs, like pensar and encender, the only other tense/mood that is effected is the Present Subjunctive. Similar to Present Indicative, the nosotros and vosotros forms are not affected by the radical change. (Naturally, since the Imperative is based off of the Present Subjunctive, these alterations follow into the Imperative.)

Pensar: piense, pienses, piense, pensemos, penséis, piensen

Encender: encienda, enciendas, encienda, encendamos, encendáis, enciendan

However, "ir" verbs that stem-change have a stem-change that occurs in the Present Subjunctive for all six forms. Note the follow three verbs which represent three distinct radical changes (e->ie, e->i, and o->ue).

Servir: sierva, siervas, sierva, sirvamos, sirváis, siervan

Pedir: pida, pidas, pida, pidamos, pidáis, pidan

Dormir: duerma, duermas, duerma, durmamos, durmáis, duerman

Note that many verbs that exhibit radical changes in the present tense (especially those that end in "er" or "ir") will have additional types of radical changes in the Preterit Tense (and all derived tenses), and the Future/Conditional Stem.