"c" for "д" in Infinitive?

I find the verb красть - украсть and looking its conjugation according with my rules of changes in verbs, I find normal that being its root крад- крад-у, -ёшь, ... the final д is suppressed before л in preterite, (and also like with т): вёд-л -> вёл вела, ... OK.
But as for Infinitive I find no rule permitting the change of "д" for "c", however for euphonic reasons some change seems rather necessary. I see the same change in вед-у, -ёшь, ... when it becomes вести.
I ask: can be taken as a rule that д of root (and perhaps even т) should be changed in Infinitive before т into c ? I have no enough vocabulary to consider more verbs that could make clear my doubt. Thanks.

Well, let's look at this:
пряду - прясть; кладу - класть
but it isn't a rule, I think:
жду - ждать; еду - ехать

And not only "д" acts that way, but sometimes also "т":
цвету - цвести; плету - плести...
and even "б":
скребу - скрести
Honestly, I don't know if there is some pattern behind all of it :pardon:

There is no pattern. Just memorize the three or four verbs like that.

Well, there probably is a rule, but since it applies to only a small handful of verbs, just memorize them. The rule is that these verbs have roots ending in a consonant and they do not have a suffix. Morphophonemically you would write them as follows:

klad-
krad-
v'od-
v'oz-
cv'et-
pl'es-
skr'ob-
Infinitive is always -ти stressed, imperative is always -и stressed or spelled ь when unstressed.
(There may be others like this, I am too lazy to look them up.)

ждать is žd-a-, root and suffix. ехать ед-у is a real anomaly. The nonpast form is
jed- (jed-u, jed'-oš, jed'-ot)
The imperative looks like it should be едь and in fact you will see this in very loose low-level or humorous speech, instead of standard Russian езжай.

Thank you. Very interesting your answers.

that's an interesting way of looking at things - working from the conjugated form back to the infinitive and trying to make a rule from that. :wacko:
блюсти, брести, обрести, пасти, трясти, fit this pattern. Looks like a general pattern. Note however грести (гребу) и скрести (скребу) with -б. In my experience, complicated rules like this may make organizing a grammar book easy but are of little use in using the language.

OK, surely you're right, but every time a make a tour on my books and exercises I try to understand why the things are like this and to find, if possible, connections between words. It is very useful (if I get it :flazhok:). My aim is to strengthen my vocabulary, and it's a long way to it. Thanks.

I believe this is a result of consonant clusters' simplification due to "syllable synharmony" and "increasing sonority" laws (законы внутрислогового сингармонизма и возрастающей звучности) in Proto-Slavic language (http://en.wikipedia.org/wiki/Proto-Slav ... synharmony). Because of these laws, consonant clusters have been simplified:

ds, ts, bs, ss, ps etc. > s:
*kradstei > *kra-dsti > кра-сти > красть (ср. крадёт)

tt, dt > st:

*plettei > *pletti > плести (ср. плетёт)
*vedti > вести (ср. ведёт)

Thank you. It's very interesting. I will add your explanation to the theme. I was considering just as properties of д, т a) change to c before т; b) dissapear before л. That gives an additional look.

Quote:

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Originally Posted by radomir

I was considering just as properties of д, т a) change to c before т; b) dissapear before л. That gives an additional look.

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I believe it is not something specific for verbs only. This is a result of _phonetical_ process happened at times when "increased sonority" rule was actual in [Proto-]Slavic.

These words are quite old; this rule isn't been working any more already for centuries, so more recent words can happily have "dt" etc.

As for "dl", "tl", the clusters mutated into "l" in Southern and Eastern Slavic languages, but remained in Western:

*mydlon > mydlo (Polish), мыло (Russian)

Thanks. Really every language has its own life along the time (as this evolution in Proto-Slavic mentioned by you), however some rules may look like being universal. Very interesting.