According to the OpenRussian database, ~72% of non-reflexive first-person singular verbs get the "-ю" suffix. The overwhelming majority of the remaining first-person singular verbs in the database get the "-у" suffix.
Furthermore, some words even get a "-ую" suffix, eg. "чувствовать" -> "чувствую".
Is there a rule-of-thumb for determining when the stem will receive a "-ю", "-у", or "-ую" suffix?
I know that the ending of the stem often dictates which suffix is used when conjugating.
That inflectional ending is simply /-u/.
When verb stem ends with /j/, then /j-/ + /-u/ results in /ju/ which is written with letter "ю" in Cyrillic alphabet.
E.g. verb име́ть has present-tense stem /imʲej-/ and past-tense stem /imʲe-/:
E.g. verb идти́ has present-tense stem /id-/ and past-tense stem /ʂed- ~ ʂ(o)-/ (/ʂed-/ is earlier form, from Proto-Slavic *šьd-):
(Infinitive is usually formed from past-tense stem, but идти́ is an exception.)
I believe source is in word infinitive:
"Идти" -- "иду"
"Любить" -- "люблю"
"Путешествовать" -- "путешествую"
"Чувствовать" -- "чувствую"
"Ковать" -- "кую"
Here is how it was explained to me, a native English speaker learning Russian from a native Russian teacher:
Regarding first person singular, for first conjugation verbs, you use ю if the stem ends with a vowel or a soft sign and you use у after consonants.
For second conjugation verbs, you use ю if the stem ends in a soft consonant or a vowel and you use у after ж, ш, щ, ч, and all hard consonants.
So it isn't exactly a simple rule of thumb but it hasn't failed me yet.
рисовать — рисую (тверд. согл.), горевать — горюю (мягк. согл.), ночевать — ночую (шипящий).
читать — читаю, уметь — умею, строить — строю, чуять — чую.
велеть — велю, гореть — горю (мягкие),
брать — беру, жить — живу (твердые),
дышать — дышу, хотеть — хочу (шипящие).
