Two sounds with similar places of articulation often merge into one sound, with the first one disappearing. In natural connected speech, there is no perceptible difference between the way that we say 'I called to help you' and 'I call to help you'. When we say 'I called to help you', we think that we're saying the 'd' at the end of 'called', and we think that we hear the 'd' when we hear someone say this - but this is simply because we know from the context that this is past tense and there ought to be a 'd' there. But in reality, we don't say it: the 'd' disappears. Both 'called to' and 'call to' sound like "calltuh".

Very good answer. It makes me more sure about something. There are some things that are not problems at all for the native English speakers, but may be big stumbling stocks for the English learners. For example, the article 'a' in the natives' natural conversations often sounds so soft that it seems not to be there to the Chinese people's ears . The 'h' sound is the same case ( dialects of most Chinese people have the sound /x/ (velar fricative) and lack the glottal fricative/ h/) . The sound linking and the resulting sound moderation are also a big problem: Chinese speakers incline to articulate every sound in sequence （Chinese language avoid sound linking, except for a few special cases）, so the English sound-linking is a big trouble for us: the word 'ear' may come in every form -- 'tear', 'mear', 'cheer', 'dear', 'fear', 'gear', 'hear', 'gear', 'near', 'peer', 'rear', and 'I' can be ' by, bye, dye, guy, high, my, .....' and even 'dyed', '-fied', '-ryed', '-live', '-mive', and so forth. These pronunciations do not cause troubles for natives, because they have rich vocabulary, and have experienced immeasurable broad living speech, so that it's easy for them to judge the 'boundaries' between the words, and the most possible words in every place, with the help of 'language sensibility'. But for an English learner, expecially one who speaks Chinese, it's never an easy pie. It's an ordinary case: a Chinese English-learner may read an article or understand the titles on the screen pretty well, but cannot comprehend them by listening, even though most of the words within are similar to them. Therefor there is a special term for this case: dumb English.

I think it's good to practice them slowly at first with careful pronunciation, but when you're ready to speed them up listen to native speakers. You'll probably be notice that there's not much separation between the 'd' and the 't' sound. It ends up sounding more like "I called'ta help you" or "I had'ta bite my tongue."

Practice different sentences out loud on your own or with a language exchange partner at the speed you are comfortable with, and you will eventually speed up.